Optical head, optical recording medium recording and/or reproducing apparatus and recording and/or reproducing method using the optical head

ABSTRACT

An optical disk drive ( 101 ) for optically recording and/or reproducing information signals is provided in which the power of a light beam emitted from an optical head ( 104 ) to an optical disk ( 102 ) is controlled by an optical-coupling efficiency varying elements ( 214, 215 ) correspondingly to the type of the optical disk, recording layer in a multilayer optical disk and a mode of operation selected while a variation of the optical-coupling efficiency is being detected by a light-detecting element ( 216 ), thereby positively varying the optical-coupling efficiency in a minimum necessary time. Thus, the power of the light beam focused on the optical disk can be varied in a wide range without having to extremely raise the ratio in output power between modes of operation at a light source ( 212 ). Therefore, even a light source whose rating of optical output power is small can be used to write and read information signals to and from any of optical disks of different types or to each of recording layers in a multilayer optical disk with a high accuracy. Namely, good characteristics in writing and reading information signals can be assured without having to largely vary the ratio in output power of the light source from the write to read mode, from one to another type of optical disks or from one to another recording layer of the multilayer optical disk.

TECHNICAL FIELD

The present invention relates to a recording and/or playback apparatusand method, for writing and/or reading various information signals toand/or from an optical recording medium such as an optical disk or thelike.

This application claims the priority of the Japanese Patent ApplicationNo. 2001-397679 filed on Dec. 27, 2001, the entirety of which isincorporated by reference herein.

BACKGROUND ART

There have so far been proposed optical recording media, typicallyoptical disks, including those of read-only type having informationsignals prerecorded therein by a pattern of pits which are microscopicholes, those having a phase-change layer to which information signalscan be written, and those having a magneto-optical recording layer towhich information signals can be recorded. Generally in an opticalrecording medium recording and/or playback apparatus that uses, as arecording medium, a recordable one among such types of optical recordingmedia, namely, in an optical disk drive, there is provided asemiconductor laser that is relatively large in maximum optical outputpower rating. It should be noted that an optical disk drive designed toplay only a read-only optical disk as a recording medium needs a lightsource not as large in maximum output rating as that of an optical diskdrive which uses a recordable optical disk but which can emit a greateramount of light than a certain value.

The reason for the above will be described below:

Generally, the semiconductor laser whose output is small cannot easilyprovide any stable light emission and the laser noise will be larger.Therefore, to assure a CNR (carrier-to-noise ratio) of informationsignals when the latter are recorded in an optical disk, the opticaloutput power of the semiconductor laser has to be set to a large levelthan a certain value (normally, 2 to 5 mW).

On the other hand, in case information signals are recorded to anoptical recording medium to which the signals can be recorded, a lightbeam is focused on the recording surface of an optical recording layerof the medium to a higher temperature than predetermined. In this case,the power of a light beam from a semiconductor laser for reading theinformation signals from the medium should be large enough to assure anample CNR of the read information signals, and also that for recordinginformation signals to the medium should be large enough to assureheating of the recording layer to the temperature higher thanpredetermined and a stable recording of the information signals.Normally, the maximum power of writing light used for writinginformation signals to an optical recording medium is about 5 to 20times larger than that of reading light. Further, for recordinginformation signals at a higher speed than a standard one, thesemiconductor laser has to provide a larger optical output power.

For the above reason, the maximum rating of optical output power of alight source used in an optical head that makes both write and read ofinformation signals to and from an optical recording medium, or of alight source used in an optical head that writes and reads informationsignals to multiple types of optical recording media, is normally about20 to 50 mW. An optical head used in an optical disk drive that rotatesan optical disk at a velocity about eight times higher than a standardvelocity of rotation to write information signals to the optical diskemploys a light source that provides an optical output power of about100 mW.

A light source having a large maximum rating of optical output power isdifficult to implement and it will consume much power. However, in casethe maximum rating of optical output power is reduced with thedifficulty and great power consumption taken in consideration, a largelaser noise will take place during playback of an optical disk, whichwill lead to a poor characteristic of reading.

On the other hand, the read-only optical disk such as DVD (digitalversatile disk) has two recording layers. Also, there has been proposedan optical disk capable of recording information signals, namely, arecordable disk, and which has two or four recording layers. Write orread of information signals to or from each of the plurality ofrecording layers of the optical disk needs a writing- or reading opticalpower more than about 1.5 to 2 times larger than write or read to orfrom a single recording layer.

Thus, in an optical disk drive which can selectively play a single-layeroptical disk and multilayer one, the ratio between the maximum opticalpower for writing information signals to the multilayer optical disk andread optical power for reading information signals from the single-layeroptical disk will be more than two times larger than that in an opticaldisk drive designed to play a single-layer optical disk.

Further, if the linear velocity at which a light beam scans a recordingtrack is different from one type to another of an optical disk beingplayed, the optical powers necessary for write and read of informationsignals also are different correspondingly. That is, as the linearvelocity at which a light beam scans a recording track on an opticaldisk is higher, the optical powers necessary for writing and readinginformation signals should be larger.

To assure a stable write or read of information signals to or from anoptical recording medium designed to have a plurality of signalrecording layers, or an optical recording medium designed to rotate at ahigher velocity, both intended for an increased recording capacity andalso to or from a conventional optical recording medium having a singlerecording layer, a light source included in an optical head should beable to provide an optical output power in a wider dynamic range.

DISCLOSURE OF THE INVENTION

Accordingly, the present invention has an object to overcome theabove-mentioned drawbacks of the related art by providing an improvedand novel optical head and an optical recording medium recording and/orplayback apparatus using the optical head.

The present invention has another object to provide an optical headshowing a good characteristic in reading information signals with asufficient suppression of reading laser noise even with a small ratio inlight source power between a write mode in which information signals arewritten to an optical recording medium and a read mode in whichinformation signals are read from the optical recording medium, and arecording/playback apparatus and method, using the optical head.

The present invention has a still another object to provide an opticalhead showing good characteristics in writing and reading informationsignals to and from an optical recording medium even with a light sourcewhose maximum rating of optical output power is small, and an opticalrecording/playback apparatus and method, using the optical head.

The present invention has a yet another object to provide an opticalhead showing good characteristics in writing and reading informationsignals, with a sufficient suppression of reading laser noise, to andfrom a recording medium, whichever the latter is one of multiple typesof optical recording media different in optimum writing and readingoptical powers of light beams from each other, an optical recordingmedium having multiple signal recording layers or an optical recordinghaving a single recording layer divided in a plurality of recordingareas, and an optical medium recording and/or playback apparatus andmethod, using the optical head.

The above object can be attained by providing an optical head including,according to the present invention, a light source; a light focusingmeans for focusing a light beams emitted from the light source onto anoptical recording medium; a beam splitting means for making the lightbeam emitted from the light source and return light coming from theoptical recording medium via the light focusing means travel alongdifferent light paths; a light detecting means for detecting the returnlight coming from the optical recording medium via the beams splittingmeans; and an optical-coupling efficiency varying means, and anoptical-coupling efficiency detecting means, provided between the lightsource and beam splitting means. The optical-coupling efficiency varyingmeans is to vary an optical-coupling efficiency that is a ratio of anamount of light focused on the optical recording medium with a totalamount of light emitted from the light source, and the optical-couplingefficiency detecting means is to detect information corresponding to anoptical-coupling efficiency varied by the optical-coupling efficiencyvarying means.

The above optical head according to the present invention can readinformation signals with a sufficient suppression of reading laser noiseeven with a small ratio in amount of light between the write and readmodes, and also read, with a sufficient suppression of reading lasernoise, information signals from an optical recording medium, whicheverthe latter is one of multiple types of optical recording media differentin optimum writing and reading optical powers of light beams from eachother, an optical recording medium having multiple signal recordinglayers or an optical recording having a single recording layer dividedin a plurality of recording areas.

Also the above object can be attained by providing an optical recordingmedium recording and/or playback apparatus which writes or readsinformation signals to a selected one of at least two or more types ofoptical recording media different in optimum recording-optical powerand/or reading optical power from each other, the apparatus including,according to the present invention, an optical head including a lightsource; and a light focusing means for focusing a light beams emittedfrom the light source onto an optical recording medium. The optical headincludes a beam splitting means for making the light beam emitted fromthe light source and return light coming from the optical recordingmedium via the light focusing means travel along different light paths;a light detecting means for detecting the return light coming from theoptical recording medium via the beams splitting means; and anoptical-coupling efficiency varying means, and an optical-couplingefficiency detecting means, provided between the light source and beamsplitting means. The optical-coupling efficiency varying means is tovary an optical-coupling efficiency that is a ratio of an amount oflight focused on the optical recording medium with a total amount oflight emitted from the light source, and the optical-coupling efficiencydetecting means is to detect information corresponding to anoptical-coupling efficiency varied by the optical-coupling efficiencyvarying means.

The above optical recording medium recording and/or playback apparatusaccording to the present invention can read information signals with asufficient suppression of reading laser noise even with a small ratio inamount of light between the write and read modes, and also read, with asufficient suppression of reading laser noise, information signals froman optical recording medium, whichever the latter is one of multipletypes of optical recording media different in optimum powers of lightbeams used for writing and reading information signals from each other,an optical recording medium having multiple signal recording layers oran optical recording having a single recording layer divided in aplurality of recording areas.

Also the above object can be attained by providing an optical recordingmedium recording and/or playback method of writing and/or readinginformation signals to a selected one of at least two or more types ofoptical recording media different in optimum recording-optical powerand/or reading optical power from each other, in which

an optical-coupling efficiency, that is a ratio of an amount of lightfocused on the optical recording medium with a total amount of lightemitted from the light source, is detected, and the optical-couplingefficiency is varied on the basis of the result of detection.

In the above optical recording medium recording and/or playback methodaccording to the present invention, information signals can be read witha sufficient suppression of reading laser noise even with the ratio inamount of light between the write and read modes, and also informationsignals can be read, with a sufficient suppression of reading lasernoise, from an optical recording medium, whichever the latter is one ofmultiple types of optical recording media different in optimum writingand reading optical powers of light beams from each other, an opticalrecording medium having multiple signal recording layers or an opticalrecording having a single recording layer divided in a plurality ofrecording areas.

These objects and other objects, features and advantages of the presentinvention will become more apparent from the following detaileddescription of the best mode for carrying out the present invention whentaken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an optical disk drive using an optical headaccording to an embodiment of the present invention.

FIG. 2 is a side elevation of an embodiment of the optical head used inthe optical disk drive according to the present invention.

FIG. 3 graphically illustrates a relation between recommended readingoptical power and writing optical power for an optical disk to be usedin the optical disk drive.

FIGS. 4A to 4D are timing diagrams showing the laser light state whichvaries when the optical disk drive is switched in mode of operationbetween write and read.

FIG. 5 shows a flow of operations made in the optical disk drive,showing that the optical disk drive being in the standby state holds apreceding light-attenuated state and selects another light-attenuatedstate after reception of a write command.

FIG. 6 sows a flow of operations made in the optical disk drive, showingthat the optical disk drive being in the standby state always holds apreceding light-attenuated state and selects another light-attenuatedstate after reception of a read command.

FIG. 7 shows a flow of operations made in the optical disk drivesupplied with a write command, showing that the optical disk drive beingin the standby state always holds a light-attenuated state in which theoptical-coupling efficiency is low and selects, only upon reception of awrite command, another light-attenuated state in which theoptical-coupling efficiency is high.

FIG. 8 shows a flow of operations made in the optical disk drivesupplied with a read command, showing that the optical disk drive beingin the standby state always holds a light-attenuated state in which theoptical-coupling efficiency is low and selects, only upon reception of aread command, another light-attenuated state in which theoptical-coupling efficiency is high.

FIG. 9 shows a flow of operations made in the optical disk drivesupplied with a write command, showing that the optical disk drive beingin the standby state always holds a light-attenuated state in which theoptical-coupling efficiency is high and selects, only upon reception ofa write command, another light-attenuated state in which theoptical-coupling efficiency is low.

FIG. 10 shows a flow of operations made in the optical disk drivesupplied with a read command, showing that the optical disk drive beingin the standby state always holds a light-attenuated state in which theoptical-coupling efficiency is high and selects, only upon reception ofa read command, another light-attenuated state in which theoptical-coupling efficiency is low.

FIG. 11 shows a flow of operations made in the optical disk drive whichselects a light-attenuated state corresponding to any of multiple typesof optical recording media.

FIG. 12 is a side elevation of another embodiment of the optical headused in the optical disk drive according to the present invention.

FIG. 13 is a side elevation of a still another embodiment of the opticalhead used in the optical disk drive according to the present invention.

FIGS. 14A to 14D are timing diagrams showing the laser light state whichvaries when the optical disk drive is switched in mode of operationbetween write and read.

FIGS. 15A and 15B are perspective views of an illustrative example of afirst type of the optical-coupling efficiency varying element used inthe optical head used in the optical disk drive according to the presentinvention.

FIGS. 16A and 16B are perspective views of another illustrative exampleof the first type of the optical-coupling efficiency varying elementused in the optical head used in the optical disk drive according to thepresent invention.

FIGS. 17A to 17C are perspective views of an illustrative example of asecond type of the optical-coupling efficiency varying element used inthe optical head used in the optical disk drive according to the presentinvention.

FIGS. 18A and 18B are perspective views of another illustrative exampleof the second type of the optical-coupling efficiency varying elementused in the optical head used in the optical disk drive according to thepresent invention.

FIGS. 19A to 19D explain together the optical-coupling efficiencyvarying elements shown in FIGS. 18A and 18B, respectively, showing theconfiguration and function of a liquid crystal element forming each ofthe optical-coupling efficiency varying elements.

FIGS. 20A and 20B are perspective views of a still another illustrativeexample of the second type of optical-coupling efficiency varyingelement in the optical head used in the optical disk drive according tothe present invention.

FIGS. 21A and 21B are perspective views of a yet another illustrativeexample of the second type of optical-coupling efficiency varyingelement in the optical head used in the optical disk drive according tothe present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The optical recording medium recording and/or playback apparatusaccording to the present invention will be described in detailconcerning an optical disk drive using an optical disk as the recordingmedium with reference to the accompanying drawings.

Referring now to FIG. 1, there is schematically illustrated in the formof a block diagram an optical disk drive according to an embodiment ofthe present invention. The optical disk drive is generally indicatedwith a reference 101. As shown, the optical disk drive 1 includes aspindle motor 103 included in a rotation drive mechanism that rotates anoptical disk 102, an optical head 104, and a feed motor 105 that movesthe optical head 104 radially of the optical disk 102 being rotated.

The spindle motor 103 is driven at a predetermined velocity under thecontrol of a system controller 107 and servo circuit 109, formingtogether a disk type discriminating means for discriminating the type ofthe optical disk 102 loaded in the optical disk drive 101.

The optical disk 102 compatible with the optical disk drive 101 includesa recordable optical disk adopting the light-intensity modulationtechnique, such as CD-R/RW, DVD-RAM, DVD-R/RW or DVD+RW, or amagneto-optical disk adopting the magneto-optical recording technique.

According to the present invention, the optical disk drive 101 isconfigured to selectively use two or more types of optical disksdifferent in optimum writing and reading optical power of light beamsfrom each other, or to use an optical disk having a recording layerthereof divided in two or more recording areas different in optimumwriting and reading optical powers from each other or an optical diskhaving a plurality of recording layers stacked one on the other.

The difference in optimum writing optical power or reading optical powerof a light beam irradiated to the signal recording layer of the opticaldisk 102 may be that caused by a difference in recording method from oneoptical disk to another or difference in rotating velocity from oneoptical disk to another. The difference in rotating velocity betweendifferent types of optical disks is a speed of a light beam scanning therecording track on an optical disk in relation to the optical disk beingrotated.

The optical disk 102 used in the optical disk drive 101 according to thepresent invention includes a multilayer optical disk having at least twoor more recording layers different or equal in optimum writing andreading optical powers of light beams from or to each other. In case amultilayer optical disk is used in the optical disk drive 101, therecording layers formed in stack in the optical disk are different inoptimum writing and reading optical powers from each other.

The light beam used to write or read information signals to or from eachof the above-mentioned optical disks has a wavelength of about 400 to780 nm.

The optical head 104 emits a light beam to the recording surface of theoptical disk 102, and detects a return light from the recording surface.The optical head 104 detects the return light from the recording surfaceof the optical disk 102, and supplies a detection output of the detectedreturn light to a preamplifier 120. The output of the preamplifier 120is sent to a signal modem/ECC block 108. The signal modem/ECC block 108modulates a signal, demodulates a modulated signal, and add an ECC(error correction code) to a signal. The optical head 104 irradiates alight beam to the recording surface of the optical disk 102 beingrotated according to a command from the signal modem/ECC block 108. Withthis irradiation of a light beam, information signals are written to orread from the optical disk 102.

The above-mentioned preamplifier 120 generates a focus error signal,tracking error signal, RF signal, etc. on the basis of a signalcorresponding to each light beam detected by the optical head 104. Theservo circuit 109, signal modem/ECC block 108 and the like performpredetermined operations such as demodulation, error correction, etc. onthe basis of the focus error signal, tracking error signal, RF signal,etc. correspondingly to the type of an optical disk used as a recordingmedium for information signals. When the optical disk 102 is intendedfor use to store data handled in information processing in a computer orthe like, for example, recorded signals thus demodulated are sent to aninformation processor such as a computer 130 via an interface 111.Namely, data recorded on the optical disk 102 is supplied as readsignals to the information processor such as a computer 130 havingconnected thereto the optical disk drive 101 according to the presentinvention.

In case the optical disk 102 loaded in the optical disk drive 101 is anaudio/video disk that records audio data and video data, the data isconverted from digital to analog by a D-A converter block of a D-A/A-Dconverter 112, and the analog data is supplied to an audio/videoprocessor 113. The signals supplied to the audio/video processor 113undergo audio/video processing therein, and transmitted to an externalimaging device/cineprojector via an audio/video signal input/output unit114.

The optical head 104 is moved by the feed motor 105 radially of theoptical disk 102 to a position corresponding to a predeterminedrecording track on the optical disk 102. At this time, the servo circuit109 controls the spindle motor 103, feed motor 105, and movement of abiaxial actuator that supports an objective lens in the optical head 104in focusing and tracking directions.

The servo circuit 109 controls an optical-coupling efficiency varyingelement disposed inside the optical head 104 according to the presentinvention to vary the optical-coupling efficiency in the optical head104, that is, a ratio of light amount focused on the optical disk 102with a total amount of light beam emitted from the laser source, fromthe write to read mode of the optical disk drive 101.

The optical disk drive 101 also includes a laser controller 121 thatcontrols a laser source included in the optical head 104. In particular,the laser controller 121 in this embodiment controls the output power ofthe laser source to be different from the write to read mode of theoptical disk drive 101.

In case the optical disk 102 loaded in the optical disk drive 101 is anyof two or more types of optical disks different, from each other, inoptical writing optical power and reading optical power of a light beamfocused on the signal recording layer, a disk type discrimination sensor115 discriminates the type of the loaded optical disk 102. It should benoted that the optical disks 102 of different types include thosedifferent in recording medium from each other, those having a recordinglayer thereof divided in a plurality of recording areas, and thosehaving a plurality of recording layers different in specification fromeach other. The optical disk 102 may be any of those adopting differentrecording methods each using the light-intensity modulation technique orvarious magneto-optical recording media. Such optical disks 102 includethose different, from each other, in optimum writing or reading opticalpower of a light beam focused on the recording layer. The disk typediscrimination sensor 115 detects the surface reflectivity of the loadedoptical disk 102, differences in shape of the optical disk 102 fromother types of optical disks, etc. to discriminate the type of theloaded optical disk 102. The detection signal from the disk typediscrimination sensor 115 is supplied to the system controller 107.

To discriminate the type of an optical disk 102 loaded in the opticaldisk drive 101, in case the optical disk 102 is encased in a cartridge,a disk type discrimination-oriented detection hole may be formed in thecartridge. Identification information for type discrimination of anoptical disk 102 may be recorded in an area for TOC (table of contents)information recorded by pre-mastered pits, grooves or the like and whichis defined along the innermost circumference of the optical disk 102. Inthis area, there are recorded disk type information indicating the typeof the optical disk 102 itself and information on recommended writingoptical power and reading optical power of a light beam used to write orread information signals to or from the optical disk 102 itself. Theseinformation are detected from the optical disk 102, and used to controlthe optical head 104 to provide writing optical power or reading opticalpower suitable for writing or reading information signals to or from theoptical disk 102.

The servo circuit 109 to control the optical-coupling efficiency iscontrolled by the system controller 107 to control the optical-couplingefficiency in the optical head 104 correspondingly to the type of theloaded optical disk 102 on the basis of the result of disk typediscrimination from the disk type discrimination sensor 115.

In case the optical disk 102 used is an optical disk having a recordinglayer thereof divided in at least two or more recording areas differentin optimum writing and reading optical powers from each other, arecording area discriminating means is used to detect an area to whichinformation signals are to be written or an area from which informationsignals are to be read. In case a plurality of recording areas isdefined concentrically with the center of the optical disk 102, theservo circuit 109 can be used as the recording area discriminatingmeans. The servo circuit 109 can discriminate a recording area to orfrom which information signals are going to be written or read bydetecting a physical relation between the optical head 104 and opticaldisk 102. It should be noted that the physical relation between theoptical head 104 and optical disk 102 may be detected on the basis of anaddress signal recorded on the optical disk 102. The servo circuit 109controls the optical-coupling efficiency in the optical head 104 on thebasis of the result of discrimination of a recording area to or fromwhich information signals are going to be written or read.

In case the optical disk 102 used in the optical disk drive 101according to the present invention is a multilayer optical disk havingat least two or more recording layers different in optimum writing andreading optical powers from each other, a recording layer discriminationmeans is used to discriminate a recording layer to which informationsignals are to be written or a recording layer from which informationsignals are to be read. The servo circuit 109 can be used as therecording layer discriminating means. The servo circuit 109 candiscriminate a recording layer to or from which information signals aregoing to be written or read by detecting a physical relation between theoptical head 104 and optical disk 102. The servo circuit 109 controlsthe optical-coupling efficiency in the optical head 104 on the basis ofthe result of discrimination of a recording layer to or from whichinformation signals are going to be written or read.

Information on the type, recording area and recording layer of eachoptical disk may be prerecorded in an area of the disk where TOC isrecorded, and the type, recording area and recording layer of theoptical disk may be discriminated by reading the information recorded inthe TOC area when the optical disk 102 is loaded in the optical diskdrive 101.

Here will be illustrated and described the optical head according to thepresent invention, used in the aforementioned optical disk drive 101.

As shown in FIG. 2, the optical head 104 includes a semiconductor laserelement 212 as light source, collimator lens 213, liquid crystal element214 and first beam splitter 215, forming together an optical-couplingefficiency varying means, second beam splitter 218, FAPC (front autopower control) detection element 219, objective lens 220, detection lens221, multi-component lens 222 and a light-detecting element 223. Theseoptical parts are mounted separately.

In the optical head 104, a light beam emitted from the semiconductorlaser element 212 is incident upon the collimator lens 213 which willprovide a parallel light beam, the parallel light beam is incident uponthe liquid crystal element 214, and then the light beam passing by theliquid crystal element 214 is incident upon the first and second beamsplitters 215 and 218 in this order. The light beam passing by the beamsplitters 215 and 218 is focused by the objective lens 220 onto thesignal recording surface of the optical disk 102.

The light beam reflected at the signal recording surface of the opticaldisk 102 is let by the second beam splitter 218 to travel along a lightpath separate from the light path along which the light beam emittedfrom the light source has traveled, and is incident upon thelight-detecting element 223 through the detection lens 221 andmulti-component lens 222. The light-detecting element 223 detects thereturn light and provides a detection signal. An RF signal, focus errorsignal, tracking error signal and the like are generated based on thedetection signal.

In the above optical head 104, after the light beam emitted from thesemiconductor laser element 212 passes by the liquid crystal element 214as the optical-coupling efficiency varying element, the latter will varythe optical-coupling efficiency appropriately. Specifically, when theoptical disk drive 101 is switched from the mode of operation from writeto read, the light beam emitted from the semiconductor laser element 212is incident upon the optical disk 102 with a smaller optical-couplingefficiency than that in the write mode on the assumption that the lightbeam is incident upon an optical disk of the same type, same recordingarea or same recording layer. Also, when the optical disk drive 101 isswitched from the read to write mode, the outgoing light beam from thesemiconductor laser element 212 is incident upon the optical disk 102with a larger optical-coupling efficiency than that in the read mode.

Note that the liquid crystal element is not limited to any one thatfunctions as a wave plate but it may be any of a twisted-nematic liquidcrystal used to form a display and the like which would be variable inpolarized state when the light beam is incident upon the beam splitter.

The semiconductor laser element 212 is supplied with a drive current I₁from the laser controller 121 in the optical head 104. The lasercontroller 121 may be disposed either outside or inside the optical head104.

The liquid crystal element 214 is varied in polarized state on the basisof a voltage applied thereto. The voltage applied to the liquid crystalelement 214 is controlled by the servo circuit 109. The light beampassing by the liquid crystal element 214 has the polarized statethereof thus varied before incidence upon the first beam splitter 215.

The first beam splitter 215 allows about 100% of P-polarized light beamsto pass by, and reflects about 100% of S-polarized light beam. When aphase difference imparted by the liquid crystal element 214 to the lightbeam is just equal to N wavelengths (N is an integer), namely, in thewrite mode of the optical disk drive 101, about 100% of the light beamis allowed to pass by the first beam splitter 215.

On the other hand, when the phase difference imparted by the liquidcrystal element 214 to the light beam is a half wave short of the Nwavelengths, namely, in the read mode, the light beam is polarized in adirection of 45 deg. from a direction in which it is normally polarized,and the first beam splitter 215 allows about 50% of the light beam topass by while reflecting the rest (about 50%).

The light beam reflected by the first beam splitter 215 is detected by asplit-light amount monitoring light-detecting element 216 as anoptical-coupling efficiency detecting means. The output of thesplit-light amount monitoring light-detecting element 216 corresponds toa product of the output power from the semiconductor laser element 212and light-split rate in the first beam splitter 215 and hence to theoptical-coupling efficiency in the optical head 104. It should be notedthat as the optical-coupling efficiency is higher, the amount of lightincident upon the split-light amount monitoring light-detecting element216 becomes smaller, while the amount of light incident upon thesplit-light amount monitoring light-detecting element 216 is larger asthe optical-coupling efficiency is lower. The amount of light incidentupon the split-light amount monitoring light-detecting element 216 isproportional to 100%—[transmittance in the optical-coupling efficiencyvarying means (%)]. The output of the split-light amount monitoringlight-detecting element 216 is sent to the preamplifier 120.

The light beam passing by the first beam splitter 215 is incident uponthe second beam splitter 218. The second beam splitter 218 splits thelight beam emitted from the semiconductor laser element 212 into a lightbeam which will actually travel toward the recording surface of theoptical disk 102 through the objective lens 220, and a light beam whichwill be incident upon the FAPC detection element 219 that monitors theamount of light beam going toward the recording surface. The output ofthe FAPC detection element 219 is sent to the laser controller 121 whichwill thus perform an operation for the auto power control. Morespecifically, the laser controller 121 controls the optical output powerof the semiconductor laser element 212 so that the output of the FAPCdetection element 219 will be as predetermined. With this control, theincident light beam output power on the surface of the optical disk 102will be constant. It should be noted that optical output power of thelight beam incident upon the optical disk 102 and controlled to thepredetermined value on the recording surface of the optical disk 102, isdifferent from the write to read mode as will further be described laterand also from one type to another of optical disk. It should also benoted that in the optical head 104 used in the optical disk drive 101adopting the light intensity modulation, the light beam is a pulsedlaser light.

The light beam emitted from the semiconductor laser element 212, splitby the beam splitter 218 and passing by the latter is incident upon theobjective lens 220. The objective lens 220 focuses the light beam comingto the optical disk 102 onto a point on the recording surface of theoptical disk 102. The objective lens 220 is moved in a focusingdirection indicated with an arrow F in FIG. 2, parallel to the opticalaxis of the objective lens 220, and in a tracking direction indicatedwith an arrow T also in FIG. 2, perpendicular to the optical axis of theobjective lens 220, respectively, according to focusing and trackingerror signals generated on the basis of a return light detected from theoptical disk 102.

The return light from the recording surface of the optical disk 102 isincident upon the second beam splitter 218 via the objective lens 220again. The second beam splitter 218 will split, by reflection, an amountof light beam corresponding to the reflectance.

The detection lens 221 converts the return light split by the secondbeam splitter 218 into a convergent light beam, the multi-component lens222 imparts an astigmatism to the convergent light for the purpose ofproducing a focus error signal by the astigmatism method, and theresultant light beam is detected by the light-detecting element 223. Afocusing error signal, tracking error signal and RF signal can beproduced on the basis of a detection output of the light-detectingelement 223.

In the optical disk drive 101 according to the present invention, thereis given a following relation:CEW>CERwhere CEW (coupling-efficiency write) is an optical-coupling efficiencyof the light beam emitted from the semiconductor laser element 212 andguided to the optical disk 102 when the optical disk drive 101 is in thewrite mode, and CER (coupling-efficiency read) is an optical-couplingefficiency of the light beam emitted from the semiconductor laserelement 212 and guide to the optical disk 102 when the optical diskdrive 101 is in the read mode.

The above is also true in case the optical-coupling efficiency of thelight beam guided to the optical disk 102 is different from one type toanother of the optical disk 102 loaded in the optical disk drive 101according to the present invention.

Therefore, by varying the optical-coupling efficiency by means of theoptical-coupling efficiency varying element from writing informationsignals to the optical disk to reading information signals from theoptical disk, or from one type to another of an optical disk loaded inthe optical disk drive, it is possible to vary the level of a light beamfocused on the recording surface of the optical disk largely from thewrite to read mode or from one type to another of the loaded opticaldisk without having to emit a light beam from the light source with anextremely large difference in output power between the write and readmodes. The optical-coupling efficiency can be varied correspondingly toa writing or reading power of the light beam focused on the signalrecording surface, which is optimum for each of different optical disktypes, each of different recording areas to and from which informationsignals are written or read or for each of different recording layers toand from which information signals are written or read. It should benoted that the relation between the optical-coupling efficiency and theoptical power of a light beam focused on the signal recording surface ofan optical disk is reversed depending upon the configuration of theoptical system in the optical head.

Thus, the optical disk drive according to the present invention can showgood characteristics in writing or reading information signals since itcan write or read the information signals to or from the signalrecording surface of an optical disk with a level of a light beam, whichis optimum for each of the modes of writing or reading the informationsignals, for each of different optical disk types, each of differentrecording areas to and from which information signals are written orread or for each of different recording layers to and from whichinformation signals are written or read.

The optical-coupling efficiency varying element included in the opticalhead according to the present invention functions as will be describedin detail below.

First, an optical-coupling efficiency (CEW) in writing informationsignals, and optical-coupling efficiency (CER) in reading informationsignals, is given as follows:CEW=CE 0×TWCER=CE 0×TRwhere CE0 is an optical-coupling efficiency in case no optical-couplingefficiency varying element is used, TW is a transmittance in theoptical-coupling efficiency varying element when the optical head is inthe signal writing mode and TR is a transmittance in theoptical-coupling efficiency varying element when the optical head is inthe signal reading mode.

A necessary light beam output (LDW) from the light source for writinginformation signals, and necessary light beam output (LDR) from thelight source for reading information signals, is given as follows:LDW=PW/CEW=PW/(CEO×TW)LDR PR/CER=PR/(CEO×TR)where PW is a necessary amount of light beam on the recording surfacefor writing information signals and PR is a necessary amount of lightbeam on the recording surface for reading information signals.

Next, a necessary dynamic range (LDW/LDR) for the optical power of thelight source is given as follows:LDW/LDR=(PW/PR)×(TR/TW)

Note that if no optical-coupling efficiency varying element is used, itmeans that TR=TW. Thus, in the optical disk drive according to thepresent invention, the necessary dynamic range for the optical power ofthe light source when the latter emits a light beam can be varieddepending upon the transmittance in the optical-coupling efficiencyvarying element.

Multiple types of optical disks different in specification from eachother are selectively usable in one optical disk drive for writing orreading information signals as will be discussed below. The multipletypes of optical disks different in specification from each otherinclude those different in recording method from each other, amultilayer optical disk having multiple signal recording layers, anoptical disk that is rotated at a high velocity for writing or readinginformation signals, etc. as having previously been described.

In an optical disk drive capable of selectively playing optical disksdifferent in specification from each other, a semiconductor laser isused as the light source included in the optical head. The semiconductorlaser provides an optical output power of 4 mW which can assure a stableemission of laser light and a sufficient suppression of laser noise, andhas a maximum optical-output rating of 60 mW.

Here, it is assumed that an amount of light beam focused on the signalrecording surface a first optical disk A designed according to a certainspecification, that is required for writing of information signalsbecause of the characteristic of the optical disk A, is PW(A), an amountof light beam focused on the signal recording surface of the opticaldisk A, required for reading information signals because of thecharacteristic of the optical disk A, is PR (A), and the required lightamounts PW(A) and PW(B) are as follows:PW(A)=20 mWPR(A)=2 mW

Also, it is assumed an amount of light beam focused on the signalrecording surface a second optical disk B designed according to acertain specification, that is required for writing of informationsignals because of the characteristic of the optical disk B, is PW(B),an amount of light beam focused on the signal recording surface of theoptical disk B, required for reading information signals because of thecharacteristic of the optical disk A, is PR(B), and the required lightamounts PW(B) and PR(B) are as follows:PW(B)=10 mWPR(B)=1 mW

In this case, the dynamic range of the optical output power of the lasersource can be given as follows if no optical-coupling efficiency varyingmeans is used:Dynamic range of light-source optical output power=60 mW/4 mW=15

The dynamic range of the required optical output power on the signalrecording surface of the optical disk can be given as follows:Dynamic range of required optical outputpower=LDW(A)/LDR(B)=PW(A)/PR(B)=20 mW/1 mW=20

That is, since the dynamic range of the light-source optical outputpower is smaller than that of the required optical output power on thesignal recording surface of the optical disk, the above light sourcewill not permit to write or read information signals accurately.

On the other hand, when the optical-coupling efficiency varying meansincluded in the optical head of the optical disk drive according to thepresent invention, the dynamic range of the optical output power will beas follows.

It is assumed here that the transmittance in the optical-couplingefficiency varying means, required for writing information signals tothe first optical disk A, is T1, and that required for readinginformation signals from the second optical disk B is T2 and thatT1=100% while T2=50%. In this case, the dynamic range of the requiredoptical output power can be given as follows:Dynamic range of required optical outputpower=LDW(A)/LDR(B)={PW(A)/PR(B)}×(R 2/T 1)=(20 mW/1 mW)×(50%/100%)=10

That is, since the dynamic range of the required optical output power issmaller than that of the light-source optical output power, a dynamicrange within the light-source optical output dynamic range will permitto write information signals to the first optical disk A and readinformation signals from the first optical disk B.

In this case, when the optical system of the optical head is designedfor the light-source optical output power to be CE0=40%, theoptical-coupling efficiency CE1 for writing information signals to thefirst optical disk A and that CE2 for reading information signals fromthe second optical disk B will have the following relation with eachother:CE 1=CE 0×T 1=40%CE 2 =CE 0×T2=20%

Therefore, the necessary light-source optical output power will be asfollows:

That is, the light-source optical output power for writing informationsignals to the first optical disk A will be:LDW(A)=PW(A)/CE 1=20 mW/40%=50 mW

The light-source optical output power for reading information signalsfrom the second optical disk B will be:LDR(B)=PR(B)/CE 2=1 mW/20%=5 mW

As above, with the light source of which the maximum rating of opticaloutput power is 60 mW, it is possible to write information signals withan optical output power of 50 mW and also to read information signalsaccurately with an optical output power of 5 mW not so larger than theoptical output power of 4 mW which allows a sufficient suppression oflaser noise.

At this time, the necessary light-source optical output power forreading information signals from the first optical disk A will be asfollows:LDR(A)=PR(A)/CE 1=2 mW/40%=5 mWLDR(A)=PR(A)/CE 2=2 mW/20%=10 mW

Also, the necessary light-source optical output power for recordinginformation signals to the second optical disk B will be as follows:LDW(B)=PW(B)/CE 1=10 mW/40%=25 mWLDW(B)=PW(B)/CE 2=10 mW/20%=50 mW

In this case, either the optical-coupling efficiency CE1 or CE2 may beused.

Note that since selection of the optical-coupling efficiency for writeor read of information signals takes a fixed length of time as will bedescribed later, the optical-coupling efficiency CE1 should be used moreconveniently for the first optical disk A and the optical-couplingefficiency CE2 be used for the second optical disk B.

With information on recommended writing and reading optical powers beingprerecorded on each optical disk used in the optical disk driveaccording to the present invention, it will be possible to write or readinformation signals to or from any optical disk with the recommendedwriting or reading optical power by controlling the writing or readingoptical power on the basis of the prerecorded information.

It is assumed here that the recommended writing and reading powers foran optical disk having a specification are PW0 and PR0, anoptical-coupling efficiency when the transmittance in theoptical-coupling efficiency varying means is about 100% is 40% whilethat when the transmittance in the optical-coupling efficiency varyingmeans is reduced is 20%, the assumed range of PW0 is 9 to 22.5 mW andrange of PR0 is 0.9 to 2.25 mW.

It is judged which of four ranges (A), (B), (C1) and (C2) as shown inFIG. 3 a combination of PR0 and PW0 read from the optical disk falls in,and a light-attenuated state, that is, a transmittance in theoptical-coupling efficiency varying means, when the optical disk driveis in the write or read mode, is set for any one, thus determined, ofthe four ranges of the recommended writing or reading optical power PR0or PW0.

That is, taking the dynamic range of a light source used in the opticalhead in consideration, the transmittance in the optical-couplingefficiency varying means has to be reduced when PP0≦1.6 mW, and also thetransmittance has to be increased when PW0≧1.2 mW.

Therefore, in case the combination of PW0 and PR0 falls in the range (A)in FIG. 3, the transmittance in the optical-coupling efficiency varyingmeans has to be reduced for the read mode. For the write mode, thetransmittance may be left as it is. Taking account of the time requiredfor selection of the optical-coupling efficiency for write or read ofinformation signals, however, the transmittance in the optical-couplingefficiency varying means should preferably be always set lower.

In case the combination of PW0 and PR0 falls in the range (B) in FIG. 3,the transmittance in the optical-coupling efficiency varying means hasto be elevated for the read mode, while the transmittance has to belowered for the write mode. So, the light-attenuated state should bechanged by switching the mode of operation between write and read.

In case the combination of PW0 and PR0 falls in the range (C1) in FIG.3, the transmittance in the optical-coupling efficiency varying meansmay be left as it is for both the write and read modes. Therefore, thetransmittance may always be high.

In case the combination of PW0 and PR0 falls in the range (C2) in FIG.3, the transmittance in the optical-coupling efficiency varying meansmay be left as it is for the read mode but has to be elevated for thewrite mode. So, taking account of the time required for selection of theoptical-coupling efficiency for write or read of information signals,the transmittance in the optical-coupling efficiency varying meansshould preferably be always set higher.

Therefore, when the combination of PW0 and PR0 falls in the ranges (C1)and (C2) in FIG. 3, the transmittance in the optical-coupling efficiencyvarying means should preferably be always high.

In case an optical disk encased in a disk cartridge is used with theoptical disk drive according to the present invention, informationindicative of a recommended writing and reading optical powerscorresponding to the optical disk in the cartridge may be provided onthe cartridge. In this case, the information indicating the recommendedwriting and reading optical powers may be formed as two holes in a partof the cartridge to identify the four ranges in FIG. 3, whereby it ispossible to make the above-mentioned operations.

Alternatively, the value of the optical-coupling efficiency mayappropriately be set within a range meeting the dynamic range of thelight source. The optical-coupling efficiency may be set to have threeor more values in some cases. In the latter case, the light source canbe produced more easily. The optical head according to the presentinvention can accommodate optical disks different in specification fromeach other and assure accurate write and read of information signalswithout having to use any special light source.

In the optical disk drive 101, selection of a multilayer optical diskhaving a plurality of recording layers formed therein and a single-layeroptical disk is controlled as will be described below.

When any of the above optical disks 102 is loaded in the optical diskdrive 101, disk discrimination data recorded in a TOC area on the loadedoptical disk 102 is read with a reading optical power smaller than theoptimum writing optical power of a light beam used for writinginformation signals to the multilayer optical disk, for example, with areading optical power of a light beam used for reading the single-layeroptical disk. In case the data thus read indicates a multilayer opticaldisk, the optical head 104 is set for writing or reading optical powerand optical-coupling efficiency suitable for the multilayer opticaldisk.

The optical disk drive 101 in which the above-mentioned multilayer andsingle-layer optical disks are selectively used is switched between thewrite and read modes as will be described below:

FIGS. 4A to 4D are timing diagrams showing the laser light state whichvaries when the optical disk drive 101 is switched in mode of operationbetween write and read. FIG. 4A shows an amount of light focused on thesignal recording surface of the optical disk 102, that is, an on-disklight power P_(P), FIG. 4B shows a transmittance P_(T) in theoptical-coupling efficiency varying element, FIG. 4C shows an outputP_(M) of the split-light amount monitoring light-detecting element 216,and FIG. 4D shows a variation of the laser output power P_(U).

In the optical disk drive 101, the laser controller 121 switches themode of operation between write W and read R with precise timingaccording to a command from the system controller 107 after the liquidcrystal element 214 starts to respond as will be described below:

More specifically, in the read mode R, the liquid crystal element 214 isapplied with an appropriate voltage by the servo circuit 109 so thatthere will occur a phase difference with which the liquid crystalelement 214 works as a half-wave plate, and the transmittance P_(T) inthe optical-coupling efficiency varying element is set to 50% as shownin FIG. 4B. At this time, the laser output power P_(U) is 5 mW as shownin FIG. 4D, and the reading characteristic is good with low laser noise.

When the optical disk drive 101 is switched in mode of operation fromread R to write W, first the servo circuit 109 varies the voltageapplied to the liquid crystal element 214 according to the command fromthe system controller 107 to change the phase difference of the liquidcrystal element 214.

With a response from the liquid crystal element 214, the transmittanceP_(T) in the optical-coupling efficiency varying element is varied from50% to 100% as shown in FIG. 4B, and the laser output power P_(U) isvaried from 5 mW to 2.5 mW as shown in FIG. 4D under the effect of theauto power control operation. At this time, the output P_(M) of thesplit-light amount monitoring light-detecting element 216 is alsolowered correspondingly to the variation of the transmittance P_(T) ofthe optical-coupling efficiency varying element and variation of thelaser output power P_(U). Since the liquid crystal element is limited inresponse speed, the power P_(P) of light focused on the optical disk isheld as a reading optical power as shown in FIG. 4A during a writepreparation W_(P), which is a transition of the response. The outputP_(M) of the split-light amount monitoring light-detecting element 216is supplied to the servo circuit 109 via the preamplifier 120. When theoutput P_(M) is lower than a preset output level reference value Poff asshown in FIG. 4C, it is determined that the transmittance P_(T) in theoptical-coupling efficiency varying element has become nearly 100% asshown in FIG. 4B, and the laser controller 121 generates a signal writepulse P_(W) according to a command supplied from the signal modem/ECCblock 108 via the system controller 107. Thus the laser output powerP_(U) is modulated as shown in FIG. 4D, and information signals will bewritten to the optical disk 102.

Next, the laser controller 121 first switches the mode of operation fromwrite W to read R according to a command from the system controller 107.In this condition, since the laser output power P_(U) is as low as 2.5mW as shown in FIG. 4D, the laser noise is high.

After the laser output power is changed to a reading optical power, theservo circuit 109 varies the voltage applied to the liquid crystalelement 214 according to a command from the system controller 107 tochange the phase difference in the liquid crystal element 214.

During a period of read preparation R_(P), of the response time of theliquid crystal element 214, the transmittance P_(T) in theoptical-coupling efficiency varying element is varied from 100% to 50%,the laser output power P_(U) is varied from 2.5 mW to 5 mW under theeffect of the auto power control operation, and thus a quality readsignal can be detected with a suppression of laser noise. At this time,when the output P_(M) of the split-light amount monitoringlight-detecting element 216 exceeds a preset reference value Pon asshown in FIG. 4C, it is determined that the optical-coupling efficiencyhas been reduced to a sufficient extent, and signal read is started. Insome cases, upon switching to the read mode R, the signal read may bestarted, and the signal read may be retried while laser noise causes anerror in the read signal. At this time, the output P_(M) of thesplit-light amount monitoring light-detecting element 216 is alsoelevated correspondingly to the variation of the transmittance P_(T) ofthe optical-coupling efficiency varying element and variation of thelaser output power P_(U) as shown in FIG. 4C.

If the switching between the write and read modes has not been effectedas above, the following trouble will take place:

First, in case the optical disk drive 101 is switched in mode ofoperation from read R to write W, the writing operation will start withthe optical output power being still high, that is, with theoptical-coupling efficiency being still small. So, if the semiconductorlaser element is operated to provide an optical output exceeding themaximum rating of optical output power of the semiconductor laserelement, the latter will possibly be damaged.

In case the mode of operation is switched from write W to read R, thereading operation will start with the optical output power being stilllow, that is, with the optical-coupling efficiency being still large.So, if the semiconductor laser element is operated to provide an opticaloutput exceeding the maximum rating of optical output power, it willpossibly be damaged. So, no good reading characteristic will be assuredbecause of much laser noise. Also, in case the optical-couplingefficiency is first reduced after completion of a writing operation, thesemiconductor laser element will possibly be damaged if it is operatedto provide an optical output exceeding the maximum rating of opticaloutput power of the semiconductor laser element.

By switching the mode of operation between write and read following theabove procedure, it is possible to sufficiently suppress the laser noisewhen reading information signals even with a small ratio between thewriting and reading optical output powers of light beams. Using a lightsource whose maximum rating of optical output power is small, theoptical disk drive can write and read information signals accurately. Toprevent the semiconductor laser element from being damaged as above,write or read should be started with precise timing and after theoptical-coupling efficiency has completely been varied by theoptical-coupling efficiency varying element, or the start and end of theoperation of varying the optical-coupling efficiency (for increase ordecrease of the optical-coupling efficiency) should be detected andcontrolled by some means before starting the write or read ofinformation signals.

The start and end of the operation of varying the optical-couplingefficiency (for increase or decrease of the optical-coupling efficiency)can be detected as follows:

For example, to vary the optical-coupling efficiency mechanically, aposition sensor or the like can be used to detect the operating state ofthe optical head. A variation of the optical output power can bedetected on the basis of an output from a rear-monitor terminal of thesemiconductor laser, namely, output from a light-detecting element thatmonitors a light beam going in a direction opposite to the normaloutgoing direction, or by detecting, by a light-detecting element, apart of a light beam not arriving at the optical disk and which is notused for writing or reading information signals.

To vary the ratio of beam splitting by a beam-splitting membrane, thelight-detecting element should be provided to detect a split opticalpower as above.

The optical disk drive constructed as above according to the presentinvention operates as will be described below with reference to the flowchart shown in FIGS. 5 to 11.

First, the output of the split-light amount monitoring light-detectingelement 216 is used to vary the optical-coupling efficiency at the timeof switching between the write and read modes as will be describedbelow. The optical disk drive 101 has three modes of operation: write,read and standby. When the write mode is represented by “W”, read modeis by “R” and standby mode is by “-”, the modes of operation are changedover in the following order:[R-W-W-R-R-R-W-R-W-R-R]

The optical-coupling efficiency is varied, namely, the light-attenuatedstate is changed in any of the following three cases, for example:

-   -   (1) The optical disk drive 101 being in the standby mode keeps a        preceding light-attenuated state, and selects another        light-attenuated state upon reception of a next command for        “read” or “write”. The operations made for this light-attenuated        state selection is shown in FIGS. 5 and 6.    -   (2) The optical disk drive 101 being in the standby mode always        holds a light-attenuated state in which the optical-coupling        efficiency is low, and selects, only upon reception of a write        command, a light-attenuated state in which the optical-coupling        efficiency is high. The operations made for this        light-attenuated state selection is shown in FIGS. 7 and 8.    -   (3) The optical disk drive 101 being in the standby mode always        holds a light-attenuated state in which the optical-coupling        efficiency is high, and selects, only upon reception of a read        command, another light-attenuated state in which the        optical-coupling efficiency is low. The operations made for this        light-attenuated state selection is shown in FIGS. 9 and 10.

The above three example cases will be described below.

In the standby mode, the optical disk drive 101 holds a precedinglight-attenuated state, and selects another light-attenuated state afterreception of a command for next “read” or “write”. First, the operationsmade in the optical disk drive 101 will be described with reference toFIGS. 5 and 6.

When the optical disk drive 101 is in the standby mode, the systemcontroller 107 controls the optical disk drive 101 hold the precedinglight-attenuated state and select another light-attenuated state afterreception of a command for next read or write. Receiving a writecommand, the system controller 107 starts up in step st1 as shown inFIG. 5. Next in step st2, the system controller 107 controls the voltageapplied to the liquid crystal element and judges whether the appliedvoltage is a voltage for a higher optical-coupling efficiency (voltagecorresponding to “Open”). When the system controller 107 has determinedthe voltage applied to the liquid crystal element to be a voltage for ahigher optical-coupling efficiency, it goes to step st3. If the systemcontroller 107 has determined the applied voltage to be a voltage for alower optical-coupling efficiency, it goes to step st4. In step st4, thesystem controller 107 controls the voltage applied to the liquid crystalelement to a voltage for a higher optical-coupling efficiency (voltagecorresponding to “Open”) and goes to step st3. In step st3, the systemcontroller 107 judges whether the output of the split-light amountmonitoring light-detecting element 216 is lower than predetermined(reference value Poff). When the system controller 107 has determinedthe output to be lower than predetermined (reference value Poff), itgoes to step st5. If the system controller 107 has determined the outputnot to be lower than predetermined (reference value Poff), it will stayin step st3. In step st5, it will judge whether the time for which thevariation of the output of the split-light amount monitoringlight-detecting element 216 has been smaller than predetermined islonger than predetermined. When the system controller 107 has determinedthe time to be longer than predetermined, it goes to step st6. If thesystem controller 107 has determined the time not to be longer thanpredetermined, it will stay in step st5. In step st6, the systemcontroller 107 has the optical disk drive 101 start a writing operation.When the optical disk drive 101 should exit the writing operation, thesystem controller 107 goes to step st7 where it will have the opticaldisk drive 101 exit the writing operation, restore the light beam outputpower to the reading power, and go back to the standby mode. In stepst8, the system controller 107 exits the operation. When the opticaldisk drive 101 is in the standby mode, the light source is ready foremission of a laser beam having a reading power.

Next, when the optical disk drive 101 is in the standby mode, the systemcontroller 107 controls the optical disk drive 101 to hold the precedinglight-attenuated state and select another light-attenuated state afterreception of a command for next read or write. When the optical diskdrive 101 receives a read command, the system controller 107 starts upin step st9 as shown in FIG. 6. Next in step st10, the system controller107 controls the voltage applied to the liquid crystal element, andjudges whether the applied voltage is a voltage for a loweroptical-coupling efficiency (voltage corresponding to “Close”). When thesystem controller 107 has determined the applied voltage to be a voltagefor a lower optical-coupling efficiency, it goes to step st11. If thesystem controller 107 has determined the applied voltage to be a voltagefor a higher optical-coupling efficiency, it goes to step st12 where itwill control the voltage applied to the liquid crystal element to avoltage for a lower optical-coupling efficiency (voltage correspondingto “Close”) and then go to step st11. In step st11, the systemcontroller 107 judges whether the output of the split-light amountmonitoring light-detecting element 216 is higher than predetermined(reference value Pon). When the system controller 107 has determined theoutput to be higher than predetermined (reference value Pon), it goes tostep st13. If the system controller 107 has determined the output not tobe higher than predetermined (reference value Pon), it will stay in stepst11. In step st13, it will judge whether the time for which thevariation of the output of the split-light amount monitoringlight-detecting element 216 has been smaller than predetermined islonger than predetermined. When the system controller 107 has determinedthe time to be longer than predetermined, it goes to step st14. If thesystem controller 107 has determined the time not to be longer thanpredetermined, it will stay in step st13. In step st14, the systemcontroller 107 has the optical disk drive 101 start a reading operation.When the optical disk drive 101 should exit the reading operation, thesystem controller 107 goes to step st15 where it will have the opticaldisk drive 101 exit the reading operation, and go back to the standbymode. In step st16, the system controller 107 exits the operation. Whenthe optical disk drive 101 is in the standby mode, the light source isready for emission of a laser beam having a reading power.

In the standby mode, the optical disk drive 101 always holds alight-attenuated state in which the optical-coupling efficiency is low,and selects another light-attenuated state only upon reception of awrite command. The operations made in the optical disk drive 101 will bedescribed with reference to FIGS. 7 and 8.

When the optical disk drive 101 is in the standby mode, the systemcontroller 107 controls the optical disk drive 101 hold alight-attenuated state in which the optical-coupling efficiency is lowand select another light-attenuated state only upon reception of a writecommand. Receiving a write command, the system controller 107 starts upin step st17 as shown in FIG. 7. Next in step st18, the systemcontroller 107 controls the voltage applied to the liquid crystalelement to a voltage for a higher optical-coupling efficiency (voltagecorresponding to “Open”), and then goes to step st19. In step st19, thesystem controller 107 judges whether the output of the split-lightamount monitoring light-detecting element 216 is lower thanpredetermined (reference value Poff). When the system controller 107 hasdetermined the output to be lower than predetermined (reference valuePoff), it goes to step st20. If the system controller 107 has determinedthe output not to be lower than predetermined (reference value Poff), itwill stay in step st19. In step st20, it will judge whether the time forwhich the variation of the output of the split-light amount monitoringlight-detecting element 216 has been smaller than predetermined islonger than predetermined. When the system controller 107 has determinedthe time to be longer than predetermined, it goes to step st21. If thesystem controller 107 has determined the time not to be longer thanpredetermined, it will stay in step st20. In step st21, the systemcontroller 107 has the optical disk drive 101 start a writing operation.When the optical disk drive 101 should exit the writing operation, thesystem controller 107 goes to step st22 where it will have the opticaldisk drive 101 exit the writing operation, restore the light beam outputpower to the reading power and then go to step st23. In step st23, thesystem controller 107 controls the voltage applied to the liquid crystalelement to a voltage for a lower optical-coupling efficiency (voltagecorresponding to “Close”) and then goes to step st24. In step st24, thesystem controller 107 shifts the optical disk drive 101 to the standbymode. In step st25, the system controller 107 exits the operation. Itshould be noted that when the optical disk drive 101 is in the standbymode, the light source is read for emission of a laser beam having areading power and the voltage applied to the liquid crystal element isfor a lower optical-coupling efficiency.

Further, when the optical disk drive 101 is in the standby mode, thesystem controller 107 controls the optical disk drive 101 to always holda light-attenuated state in which the optical-coupling efficiency islow, and select, only upon reception of a write command, anotherlight-attenuated state in which the optical-coupling efficiency is high.When the optical disk drive 101 receives a read command, the systemcontroller 107 starts up in step st26 as shown in FIG. 8. Next in stepst27, the system controller 107 judges whether the output of thesplit-light amount monitoring light-detecting element 216 is higher thanpredetermined (reference value Pon). When the system controller 107 hasdetermined the output to be higher than predetermined (reference valuePon), it goes to step st28. If the system controller 107 has determinedthe output to be lower than predetermined (reference value Pon), it willstay in step st27. In step st28, it will judge whether the time forwhich the variation of the output of the split-light amount monitoringlight-detecting element 216 has been smaller than predetermined islonger than predetermined. When the system controller 107 has determinedthe time to be longer than predetermined, it goes to step st29. If thesystem controller 107 has determined the time not to be longer thanpredetermined, it will stay in step st28. In step st29, the systemcontroller 107 has the optical disk drive 101 start a reading operation.When the optical disk drive 101 should exit the reading operation, thesystem controller 107 goes to step st30 where it will have the opticaldisk drive 101 exit the reading operation, and go back to the standbymode. In step st31, the system controller 107 exits the operation. Whenthe optical disk drive 101 is in the standby mode, the light source isready for emission of a laser beam having a reading power and thevoltage applied to the liquid crystal element is for a loweroptical-coupling efficiency.

In the standby mode, the optical disk drive 101 always holds alight-attenuated state in which the optical-coupling efficiency is high,and selects, only upon reception of a read command, anotherlight-attenuated state in which the optical-coupling efficiency is low.The operations made in the optical disk drive 101 will be described withreference to FIGS. 9 and 10.

When the optical disk drive 101 is in the standby mode, the systemcontroller 107 controls the optical disk drive 101 hold alight-attenuated state in which the optical-coupling efficiency is highand select, only upon reception of a read command, anotherlight-attenuated state in which the optical-coupling efficiency is low.Upon reception of a write command, the system controller 107 starts upin step st32 as shown in FIG. 9. Next in step st33, the systemcontroller 107 judges whether the output of the split-light amountmonitoring light-detecting element 216 is lower than predetermined(reference value Poff). When the system controller 107 has determinedthe output to be lower than predetermined (reference value Poff), itgoes to step st34. If the system controller 107 has determined theoutput not to be lower than predetermined (reference value Poff), itwill stay in step st33. In step st34, it will judge whether the time forwhich the variation of the output of the split-light amount monitoringlight-detecting element 216 has been smaller than predetermined islonger than predetermined. When the system controller 107 has determinedthe time to be longer than predetermined, it goes to step st35. If thesystem controller 107 has determined the time not to be longer thanpredetermined, it will stay in step st34. In step st35, the systemcontroller 107 has the optical disk drive 101 start a writing operation.When the optical disk drive 101 should exit the writing operation, thesystem controller 107 goes to step st36 where it will have the opticaldisk drive 101 exit the writing operation, restore the light beam outputpower to the reading power and go back to the standby mode. In stepst37, the system controller 107 exits the operation. It should be notedthat when the optical disk drive 101 is in the standby mode, the lightsource is read for emission of a laser beam having a reading power andthe voltage applied to the liquid crystal element is for a higheroptical-coupling efficiency.

Next, when the optical disk drive 101 is in the standby mode, the systemcontroller 107 controls the optical disk drive 101 to always hold alight-attenuated state in which the optical-coupling efficiency is high,and select, only upon reception of a read command, anotherlight-attenuated state in which the optical-coupling efficiency is low.Upon reception of a read command, the system controller 107 starts up instep st38 as shown in FIG. 10. Next in step st39, the system controller107 controls the voltage applied to the liquid crystal element to avoltage for a lower optical-coupling efficiency (voltage correspondingto “Close”) and goes to step st40. In step st40, the system controller107 judges whether the output of the split-light amount monitoringlight-detecting element 216 is higher than predetermined (referencevalue Pon). When the system controller 107 has determined the output tobe higher than predetermined (reference value Pon), it goes to stepst41. If the system controller 107 has determined the output not to behigher than predetermined (reference value Pon), it will stay in stepst40. In step st41, it will judge whether the time for which thevariation of the output of the split-light amount monitoringlight-detecting element 216 has been smaller than predetermined islonger than predetermined. When the system controller 107 has determinedthe time to be longer than predetermined, it goes to step st42. If thesystem controller 107 has determined the time not to be longer thanpredetermined, it will stay in step st41. In step st42, the systemcontroller 107 has the optical disk drive 101 start a reading operation.When the optical disk drive 101 should exit the reading operation, thesystem controller 107 goes to step st43 where it will have the opticaldisk drive 101 exit the reading operation, and then goes to step S44. Instep st44, the system controller 107 controls the voltage applied to theliquid crystal element to a voltage for a higher optical-couplingefficiency (voltage corresponding to “Open”) and goes to step st45. Instep st45, the system controller 107 have the optical disk drive 101 goback to the standby mode. In step st46, it exits the operation.

Also, to write or read information signals to or from any of multipletypes of optical disks different in specification from each other, thesystem controller 107 starts up in step st47 as shown in FIG. 11. Nextin step st48, the system controller 107 controls the voltage applied tothe liquid crystal element to set, as a voltage for a loweroptical-coupling efficiency (voltage corresponding to “Close”), theoptical output power of the light beam focused on the recording surfaceof the optical disk (on-disk power) to a predetermined value, forexample, 0.9 mW, and then goes to step st49. In step st49, the systemcontroller 107 judges whether the output of the split-light amountmonitoring light-detecting element 216 is higher than predetermined(reference value Pon (corresponding to 0.9 mW)). It should be noted thatsince the output of the split-light amount monitoring light-detectingelement 216 varies correspondingly to a setting of the on-disk power, sothe value of the setting (reference value Pon) has to be appropriatelyset correspondingly. When the system controller 107 has determined theof the split-light amount monitoring light-detecting element 216 to behigher than predetermined (reference value Pon), it goes to step st50.If the system controller 107 has determined the output not to be higherthan Pon, it will stay in step st49. In step st50, the system controller107 judges whether the time for which the variation of the output of thesplit-light amount monitoring light-detecting element 216 has beensmaller than predetermined is longer than predetermined. When the systemcontroller 107 has determined the time to be longer than predetermined,it goes to step st51. If the system controller 107 has determined thetime not to be longer than predetermined, it will stay in step st50. Instep st51, the system controller 107 has the optical head start a focusservo operation (focus ON) and goes to step st52. In step st52, thesystem controller 107 moves the optical head to the innermostcircumference of the optical disk, and goes to step st53. In step st53,the system controller 107 detects a recommended writing power PW0 andrecommended reading power PR0, and goes to step st54.

In step st54, the system controller 107 judges whether the recommendedreading power PR0 is smaller than predetermined, for example, 1.6 mW.When the system controller 107 has determined the recommended readingpower PR0 to be smaller than predetermined, it goes to step st55. If thesystem controller 107 has determined the recommended reading power PR0not to be smaller than predetermined, it goes to step st58. In stepste55, the system controller 107 judges whether the recommended writingpower PW0 is larger than predetermined, for example, 12 mW. When thesystem controller 107 has determined the recommended writing power PW0to be larger than predetermined, it goes to step st56. If the systemcontroller 107 has determined the recommended writing power PW0 not tobe larger than predetermined, it goes to step st57.

In step st56, the system controller 107 determines the light-attenuatedstate to be (A) as shown in FIG. 3, and controls the voltage applied tothe liquid crystal element according to the result of determination. Instep st57, the system controller 107 determines the light-attenuatedstate to be (B) as in FIG. 3, and controls the voltage for applicationaccording to the result of determination. In step st58, the systemcontroller 107 determines the light-attenuated state to be (C) (=C1 orC2) as in FIG. 3, and controls the voltage for application according tothe result of determination.

In the optical head 104 according to the present invention, theoptical-coupling efficiency varying means may be formed from opticalelements provided on the light path between the light source and abeam-splitting means, and the beam-splitting means, as shown in FIG. 12.

As shown in FIG. 12, the optical head 104 includes the semiconductorlaser element 212, collimator lens 213, liquid crystal element (of avariable polarized state type) 234, and first beam splitter 218, secondbeam splitter 224, FAPC (front auto power control) detection element219, objective lens 220, detection lens 221, a multi-component lens 222and light-detecting element 223. The optical head 104 is formed fromthese optical parts mounted separately.

The semiconductor laser element 212 is supplied with a drive current 12from the laser controller 121 of the optical head 104. The voltageapplied to the liquid crystal element 214 is controlled by the servocircuit 109. It should be noted that the laser controller 121 may beprovided outside the optical head 104 or mounted in the latter.

In the optical head 104 shown in FIG. 12, the laser beam emitted fromthe semiconductor laser element 212 is incident upon the collimator lens221 where it will be parallelized, and this parallel beams is incidentupon the liquid crystal element 234.

The liquid crystal element 234 varies in polarized state depending uponthe applied voltage. The light beam passing by the liquid crystalelement 234, having been varied in polarized state, is incident upon thefirst beam splitter 218.

The first beam splitter 218 allows about 100% of the P-polarized lightto pass by and reflects about 100% of the S-polarized light. When thephase difference imparted by the liquid crystal element 234 is just Nwavelengths (N is an integer), about 100% of the light beam passes bythe first beam splitter 218.

On the other hand, when the phase difference imparted by the liquidcrystal element 234 is a half wavelength short of the N wavelengths, thelight beam has the polarized direction thereof rotated 45 deg. from adirection in which it is normally polarized so that about 50% thereofwill pass by the first beam splitter 218 while the rest (about 50%) isreflected.

The light beam reflected by the first beam splitter 218 is incident uponthe second beam splitter 224. The light reflected by the second beamsplitter 224 is detected by the split-light amount monitoringlight-detecting element 216. The output of the split-light amountmonitoring light-detecting element 216 corresponds to a product of theoutput power from the semiconductor laser element 212 and light-splitrate in the first beam splitter 218, and hence it generally correspondsto the optical-coupling efficiency in the optical head 104. It should benoted that as the optical-coupling efficiency is higher, the amount oflight incident upon the split-light amount monitoring light-detectingelement 216 becomes smaller, while the amount of light incident upon thesplit-light amount monitoring light-detecting element 216 is larger asthe optical-coupling efficiency is lower. The amount of light incidentupon the split-light amount monitoring light-detecting element 216 isproportional to 100%—[transmittance in the optical-coupling efficiencyvarying means (%)]. The output of the split-light amount monitoringlight-detecting element 216 is supplied to the preamplifier 120.

The light beam passing by the second beam splitter 224 is incident uponthe FAPC light-detecting element 219 that monitors the amount of thelight beam going to the signal recording surface of the optical disk102. The output of the FAPC detection element 219 is sent to the lasercontroller 121 which will thus perform an operation for the front autopower control (FAPC). More specifically, the laser controller 121controls the optical output power of the semiconductor laser element 212so that the output of the FAPC detection element 219 will have apredetermined value. With this control, the incident light beam outputon the surface of the optical disk 102 will be constant. It should benoted that the incident light beam output upon the recording surface ofthe optical disk 102, controlled to the predetermined value, isdifferent from the write to read mode as having previously beendescribed and also from one type to another of optical disk. It shouldalso be noted that in the optical head 104 used in the optical diskdrive 101 adopting the light intensity modulation, the light beam is apulsed laser light.

The light beam emitted from the semiconductor laser element 212, splitby the first beam splitter 218 and passing by the latter is incidentupon the objective lens 220. The objective lens 220 focuses the lightbeam coming to the optical disk 102 onto a point on the recordingsurface of the optical disk 102. The objective lens 220 is moved in afocusing direction indicated with an arrow F in FIG. 12, parallel to theoptical axis of the objective lens 220, and in a tracking directionindicated with an arrow T also in FIG. 12, perpendicular to the opticalaxis of the objective lens 220, respectively, according to focusing andtracking error signals generated on the basis of a return light detectedfrom the optical disk 102.

The return light from the recording surface of the optical disk isincident upon the first beam splitter 218 through the objective lens 220again. The first beam splitter 218 splits, by reflection, the returnlight in an amount of light corresponding to the reflectance of thelatter.

The detection lens 221 converts the return light split by the first beamsplitter 218 into a convergent light beam, the multi-component lens 222imparts an astigmatism to the convergent light for the purpose ofproducing a focus error signal by the astigmatism method, and thelight-detecting element 223 detects the resultant light beam. A focusingerror signal, tracking error signal and RF signal can be produced on thebasis of a detection output of the light-detecting element 223.

Note that in the optical head 104 according to the present invention,the optical-coupling efficiency varying means may be formed from opticalelements provided on the light path between the light source and abeam-splitting means, and the beam-splitting means, as shown in FIG. 13.The optical head 104 includes the semiconductor laser element 212,collimator lens 213, liquid crystal element (of an independenttransmittance-variable type) 254 as the optical-coupling efficiencyvarying means, and first beam splitter 218, FAPC (front auto powercontrol) detection element 219, objective lens 220, detection lens 221,a multi-component lens 222 and light-detecting element 223. The opticalhead 104 is formed from these optical parts mounted separately.

The semiconductor laser element 212 is supplied with a drive current 13from the laser controller 121 of the optical head 104. The voltageapplied to the liquid crystal element 214 is controlled by the servocircuit 109. It should be noted that the laser controller 121 may beprovided outside the optical head 104 or mounted in the latter.

The output B_(P) of the rear-monitoring photodetector (not shown)included in the semiconductor laser element 212 is supplied to thepreamplifier 120. The voltage applied to the liquid crystal element 254of the independent transmittance-variable type is controlled by theservo circuit 109.

In the optical head 104 shown in FIG. 13, the laser beam emitted fromthe semiconductor laser element 212 is incident upon the collimator lens221 where it will be parallelized, and this parallel beams is incidentupon the liquid crystal element 254.

The liquid crystal element 254 varies in polarized state depending uponthe applied voltage. The light beam passing by the liquid crystalelement 254, having been varied in polarized state, is incident upon thefirst beam splitter 218.

The first beam splitter 218 allows about 100% of the P-polarized lightto pass by and reflects about 100% of the S-polarized light. Since theliquid crystal element 254 imparts no phase difference to the lightbeam, about 100% of the light beam will pass by the first beam splitter218.

The small amount of light beam reflected by the first beam splitter 218is incident upon the FAPC light-detecting element 219 that monitors theamount of the light beam going to the signal recording surface of theoptical disk 102. The output of the FAPC detection element 219 is sentto the laser controller 121 which will thus perform an operation for thefront auto power control (FAPC). More specifically, the laser controller121 controls the optical output power of the semiconductor laser element212 so that the output of the FAPC detection element 219 will have apredetermined value. With this control, the incident light beam outputon the surface of the optical disk 102 will be constant. The output ofthe rear-monitoring photodetector will be proportional to the opticaloutput power of the semiconductor laser element 212.

It should be noted that the incident light beam output upon therecording surface of the optical disk 102, controlled to thepredetermined value, is different from the write to read mode as havingpreviously been described and also from one type to another of opticaldisk. It should also be noted that in the optical head 104 used in theoptical disk drive 101 adopting the light intensity modulation, thelight beam is a pulsed laser light.

The light beam emitted from the semiconductor laser element 212, passingby the first beam splitter 218, is incident upon the objective lens 220.The objective lens 220 focuses the light beam coming to the optical disk102 onto a point on the recording surface of the optical disk 102. Theobjective lens 220 is moved in a focusing direction indicated with anarrow F in FIG. 13, parallel to the optical axis of the objective lens220, and in a tracking direction indicated with an arrow T also in FIG.13, perpendicular to the optical axis of the objective lens 220,respectively, according to focusing and tracking error signals generatedon the basis of a return light detected from the optical disk 102.

The return light from the recording surface of the optical disk isincident upon the first beam splitter 218 through the objective lens 220again. The first beam splitter 218 splits, by reflection, the returnlight in an amount of light corresponding to the reflectance of thelatter.

The detection lens 221 converts the return light split by the first beamsplitter 218 into a convergent light beam, the multi-component lens 222imparts an astigmatism to the convergent light for the purpose ofproducing a focus error signal by the astigmatism method, and thelight-detecting element 223 detects the resultant light beam. A focusingerror signal, tracking error signal and RF signal can be produced on thebasis of a detection output of the light-detecting element 223.

In the optical disk drive using the optical head 104 constructed asshown in FIG. 13, selection between the write and read modes is effectedas will be described below with reference to FIGS. 14A to 14D.

FIGS. 14A to 14D are timing diagrams showing the laser light state whichvaries when the optical disk drive 101 is switched in mode of operationbetween write W and read R. FIG. 14A shows an amount of light focused onthe signal recording surface of the optical disk 102, that is, anon-disk light power P_(P), FIG. 14B shows a transmittance P_(T) of thelaser beam in the optical-coupling efficiency varying element, FIG. 14Cshows the level of output B_(P) of the rear monitor, and FIG. 14D showsa variation of the laser output power P_(U).

Also the optical disk drive using the optical head 104 according to thisembodiment can be controlled similarly to the optical disk drive 101using the optical head 104 constructed as shown in FIG. 2 by settingreference levels of the reference values Pon and Poff for therear-monitor output B_(P) as shown in FIG. 14C.

In the optical disk drive 101 using the optical head 104 shown in FIG.13, the laser controller 121 makes a selection between the write mode Wand read mode R with precise timing according to a command from thesystem controller 107 after the liquid crystal element 214 starts torespond as will be described below.

More specifically, in the read mode R, the voltage applied to the liquidcrystal element 254 is adjusted by the servo circuit 109 for thetransmittance P_(T) to be 50% as shown in FIG. 14B. At this time, thelaser output power P_(U) is 5 mW as shown in FIG. 14D, and the readingcharacteristic is good with less laser noise.

When the optical disk drive 101 is switched in mode of operation fromread R to write W, first the servo circuit 109 varies the voltageapplied to the liquid crystal element 254 according to the command fromthe system controller 107 to change the transmittance P_(T) in theliquid crystal element 254 as shown in FIG. 14B.

During a period W_(P) of write preparation, of the response time of theliquid crystal element 254, the transmittance P_(T) in the liquidcrystal element 254 is varied from 50% to 100% as shown in FIG. 14B, andthe laser output power P_(U) is varied from 5 mW to 2.5 mW as shown inFIG. 14D under the effect of the auto power control operation. At thistime, the rear-monitor output B_(P) is also lowered correspondingly tothe variation of the transmittance P_(T) of the liquid crystal element254 and variation of the laser output power P_(U), as shown in FIG. 14C.Since the liquid crystal element is limited in response speed, theoptical power of light focused on the signal recording surface of theoptical disk 102 is held as a reading optical power as shown in FIG. 4Aduring a write preparation W_(P), which is a transition of the response.The rear-monitor output B_(P) is supplied to the servo circuit 109 viathe preamplifier 120. When the output B_(P) is lower than a presetoutput level reference value Poff, it is determined that thetransmittance P_(T) in the liquid crystal element 254 has become nearly100%, and the laser controller 121 generates a signal write pulse P_(W)according to a command supplied from the signal modem/ECC block 108 viathe system controller 107. Thus the laser output power P_(U) ismodulated, and information signals will be written to the optical disk102.

The optical disk drive 101 is switched from the write mode W to readmode R as will be described below.

For changeover from the write mode W to read mode R, the lasercontroller 121 first switches the mode of operation from write W to readR according to a command from the system controller 107. In thiscondition, namely, in the initial state of the read preparation R_(P),since the laser output power P_(U) is as low as 2.5 mW as shown in FIG.14D, the laser noise is high.

After the laser output power is changed to a reading optical power, theservo circuit 109 varies the voltage applied to the liquid crystalelement 254 according to a command from the system controller 107 tochange the transmittance in the liquid crystal element 254.

During a period of read preparation R_(P), of the response time of theliquid crystal element 254, the transmittance P_(T) in the liquidcrystal element 254 is varied from 100% to 50% as shown in FIG. 14B, thelaser output power P_(U) is varied from 2.5 mW to 5 mW under the effectof the auto power control operation, and thus a quality read signal canbe detected with a suppression of laser noise, as shown in FIG. 14D. Atthis time, when the rear-monitor output B_(P) exceeds a preset referencevalue Pon as shown in FIG. 14C, it is determined that the transmittanceP_(T) in the liquid crystal element 254 has been reduced to a sufficientextent, and signal read is started in the read mode R for readinginformation signals from the optical disk 102. In some cases, uponswitching to the read mode R, the signal read may be started, and modeswitching may be repeatedly done if a laser noise causes an error in theread signal. At this time, the rear-monitor output B_(P) is alsoelevated correspondingly to the variation of the transmittance P_(T) inthe liquid crystal element 254 and variation of the laser output powerP_(U) as shown in FIG. 14B.

If the switching between the write and read modes W and R has not beeneffected as above, the following trouble will take place:

First, in case the optical disk drive 101 is switched in mode ofoperation from read R to write W, the writing operation will start withthe optical output power being still high, that is, with theoptical-coupling efficiency being still small. So, if the semiconductorlaser element is operated to provide an optical output exceeding themaximum rating of optical output power of the semiconductor laserelement, the latter will possibly be damaged.

In case the mode of operation is switched from write W to read R, thereading operation will start with the optical output power being stilllow, that is, with the optical-coupling efficiency being still large.So, no good reading characteristic will be assured because of much lasernoise. Also, if the optical-coupling efficiency is first reduced aftercompletion of a writing operation, the semiconductor laser element willpossibly be damaged as the case may be if it is operated to provide anoptical output power exceeding the maximum rating of optical outputpower.

On this account, by switching the mode of operation between write W andread R following the above procedure, it is possible to sufficientlysuppress the laser noise when reading information signals even with asmall ratio between the writing and reading optical output powers oflight beams. Using a light source whose maximum rating of optical outputpower is small, the optical disk drive can write and read informationsignals accurately.

Note that in each of the aforementioned optical heads, theoptical-coupling efficiency varying means may use a wave plate typeliquid crystal element as the liquid crystal element or may be of anyother type.

The optical-coupling efficiency varying means will be described indetail below concerning some types thereof.

The first type of the optical-coupling efficiency varying element usedin the optical head according to the present invention uses a meanscapable of varying the transmittance or reflectance of a light beam.That is, the means changes the optical-coupling efficiency by varyingthe transmittance or reflectance of a light beam.

The second type of the optical-coupling efficiency varying element usesa light-path branching means that splits a light beam into at least twobeams and makes them beams travel along different light paths. Namely,the light-path branching means varies the optical-coupling efficiency byvarying the ratio of beam splitting between the two light paths.

Each of the above types of the optical-coupling efficiency varyingelements will be described in detail below.

FIGS. 15A and 15B are perspective views of an illustrative example ofthe first type of the optical-coupling efficiency varying element. Thisoptical-coupling efficiency varying element uses a transmission typeliquid crystal element 21 capable of varying the transmittance of alight beam. With changing of a voltage applied to the liquid crystalelement 21, the latter varies the transmittance of a light beam L₁. Morespecifically, with a drive voltage applied to the liquid crystal element21 being changed, the orientation of the liquid crystal is varied tochange the light transmittance level from high as in FIG. 15A to low asin FIG. 15B. The liquid crystal element 21 is driven under the controlof a liquid crystal drive circuit provided in the servo circuit 109.

FIGS. 16A and 16B are perspective views of another illustrative exampleof the first type of the optical-coupling efficiency varying element.This optical-coupling efficiency varying element uses a filter plate 22capable of varying the transmittance of a light beam. The filter plate22 has a transparent plate 22 a slidable in the direction of arrow S inFIGS. 16A and 16B and a translucent filter 22 b, for example, providedin a part of the transparent plate 22 a.

The filter plate 22 shown in FIGS. 16A and 16B displaces the filter 22 bin the direction of arrow S in FIG. 16 on the light path along which thelight beam L₁ travels to select either a state in which the light beamL₁ passes by a part of the transparent plate 22 a as shown in FIG. 16Aand a state in which the light beam L₁ passes by the filter 22 b asshown in FIG. 16B, and varies the transmittance of the light beam L₁.

More specifically, it is possible to reduce the passing light beam andhence the optical-coupling efficiency by positioning the filter 22 b onthe light path of the light beam L₁ as shown in FIG. 16B. Also, it ispossible to allow all the light beam L₁ to pass by to increase theamount of passing light and hence elevate the optical-couplingefficiency by positioning the transparent plate 22 a on the light pathof the light beam L₁ as shown in FIG. 16A.

The filter plate 22 is supported on a piezoelectric element or the like,for example, and the piezoelectric element is controlled by a drivecircuit included in the servo circuit 109 to control the position of thefilter plate 22. Alternatively, the filter plate 22 may be supported bya mechanism including a screw and motor, and the filter plate 22 bepositioned under the control of a drive circuit included in the servocircuit 109.

The aforementioned optical-coupling efficiency varying element isconfigured to change the transmittance of a light beam, but it may beconfigured with a reflective element provided on the light path of thelight beam L₁ to change the reflectance of the light beam L₁.

FIGS. 17A and 17B are perspective views of an illustrative example of asecond type of the optical-coupling efficiency varying element. Asshown, this optical-coupling efficiency varying element uses a waveplate 31 and beam splitter 32, provided to branch the light path of thelight beam L₁. When the wave plate 31 is turned circumferentially of thelight path of the light beam L₁, the latter is split into light beams,and the light beams are made to travel along different light paths, by abeam-splitting membrane 32 a included in the beam splitter 32.

As shown in FIG. 37A, when the optical-axial direction P_(L) of the waveplate 31 is aligned with the polarized direction P₁ of incident lightL₃, the latter is not reflected by the beam splitter 32, but whollypasses by the beam splitter 32.

On the other hand, when the optical-axial direction P₁ of the wave plate31 is turned a fixed angle α from the polarized direction P_(L) of theincident light L₃ as shown in FIG. 17B, the beam splitter 32 reflects apart L₃′ of the incident light L₃ and allows only the remainder LL₃ ofthe incident light L₃ to pass by in the direction of the optical disk.

For example, in case the beam-splitter membrane is a PS(perfect-splitting) membrane (T_(P)=100%, R_(S)=100%) and the wave plate31 is a half-wave plate, the rotation angle α and transmittance ratio Thave the following relation between them.

First, the polarized direction is turned 2α with a rotation angle α. Atthis time, the ratio of P-polarized light incident upon the beamsplitter 32, that is, the transmittance ratio T, is given as follows asshown in FIG. 17C.T=cos 22α=(1+cos 4α)/2

Therefore, for an optical-coupling efficiency of 100 to 50%, thepolarized direction should be turned between angles of α=0 deg. andα=22.5 deg. Thus, the polarized direction varies 45 deg, and thetransmittance ratio can be controlled to 100% or 50%.

FIGS. 18A and 18B are perspective views of another illustrative exampleof the second type of the optical-coupling efficiency varying element.This optical-coupling efficiency varying element includes a liquidcrystal element 33 and beam splitter 34, provided to split the lightbeam L₁ into light beams and make the light beams travel along differentlight paths. When the liquid crystal element 33 works as a wave plate,and the beam splitter 34 splits a light beam by a beam-splittingmembrane 34 a included in the beam splitter 34.

More specifically, the liquid crystal element 33 used in thisoptical-coupling efficiency varying element has a rubbing directionP_(R) set to 22.5 deg. as shown in FIG. 18A. By changing the phasedifference of the liquid crystal element 33 to Nλ to (N+0.5)λ or Nλ to(N−0.5)λ (where N is an integer and λ is a wavelength), the polarizeddirection of a light beam L₄ incident upon the beam splitter 34 can bechanged 45 deg. and the transmittance ratio be changed within a range of100% to 50%.

Also, the liquid crystal element 33 used in the optical-couplingefficiency varying element has a rubbing direction P_(R) set to 45 deg.as shown in FIG. 18B. By changing the phase difference of the liquidcrystal element 33 to Nλ to (N+0.25)λ or Nλ to (N−0.25)λ (where N is aninteger and λ is a wavelength), the polarized direction of a light beamLP_(R) incident upon the beam splitter 34 can be changed from aP-polarized light to a circularly polarized light and the transmittanceratio be changed within a range of 100% to 50%.

Now, the liquid crystal element develops a phase difference on the basisof the principle which will be described below:

FIGS. 19A and 19B are sectional views of the liquid crystal elements,FIG. 19C explains a variation in refractive index of the liquid crystalelement when applied with a voltage, and FIG. 19D shows a variation ofthe phase difference in relation to a voltage applied to the liquidcrystal element.

As shown in FIGS. 19A and 19B, a liquid crystal element, generallyindicated with a reference 40, includes two glass substrates 41 and 42,liquid crystal molecules 49 sealed between the glass substrates 41 and42 and oriented by orientation membranes 43 and 44 provided on the innersurfaces of the glass substrates 41 and 42, respectively, transparentelectrode membranes 45 and 46 provided between the glass substrate 41and orientation membranes 43 and between the glass substrate 42 andorientation membrane 44, respectively. By varying the voltage appliedbetween the transparent electrode membranes 45 and 46, the liquidcrystal molecules 49 are changed from a state in which they are disposedalong a rubbing direction indicated with an arrow A in FIG. 19A,parallel to the orientation membranes 43 and 44, as shown in FIG. 19A toa state in which they are erected vertically in relation to theorientation membranes 43 and 44 as shown in FIG. 19B.

It is assumed here that the refractive index in a direction along therubbing direction when the liquid crystal molecules 49 are parallel tothe orientation membranes 43 and 44 is N1 and the refractive index in adirection along the rubbing direction when the liquid crystal molecules49 are perpendicular to the rubbing direction is N2. The refractiveindex N1 in the direction along the rubbing direction changes as shownin FIG. 19C correspondingly to displacement of the liquid crystalmolecules 49, caused by a variation of the applied voltage. It should benoted that the refractive index N2 in the direction perpendicular to therubbing direction remains constant.

As a result, a phase difference developed in an incident light L5traveling in the direction along the rubbing direction is varied asshown in FIG. 19D.

The above principle permits to use the liquid crystal element as a waveplate. By combining the liquid crystal element with a beam splitter, alight-path branching means can be implemented.

Note that the examples of the optical-coupling efficiency varyingelement in FIGS. 18A and 18B are just typical illustrative ones andhence the ranges of the rubbing direction and phase difference can beset variably correspondingly to a necessary variation of thetransmittance ratio.

The liquid crystal element is not limited to any one which can serve asa wave plate but may be formed from a liquid crystal which would be ableto vary the polarized light incident upon the beam splitter, such as atwisted-nematic type liquid crystal used in a liquid crystal display.

FIGS. 20A and 20B are perspective views of a still another illustrativeexample of the second type of optical-coupling efficiency varyingelement. This optical-coupling efficiency varying element uses adiffraction grating plate 35 as a light-path branching means.

The diffraction grating plate 35 has a transparent plate 35 a slidablein the direction of arrow S in FIGS. 20A and 20B and a diffractiongrating 35 b provided in a part of the transparent plate 35 a.

The diffraction grating plate 35 shown in FIGS. 20A and 20B displacesthe diffraction grating 35 b in the direction of arrow S in FIG. 20A onthe light path along which the light beam L₆ travels to select either astate in which the light beam L₆ passes by a part of the transparentplate 35 a as shown in FIG. 20A and a state in which the light beam L₆passes by the diffraction grating 35 b as shown in FIG. 20B, and variesthe split state of the light beam L₆.

More specifically, it is possible to reduce the optical-couplingefficiency by positioning the diffraction grating 35 b on the light pathof the light beam L₆ as shown in FIG. 20B to split the laser beam L₆.

Also, it is possible to increase the optical-coupling efficiency bypositioning a part, except for the diffraction grating 35 b, of thetransparent plate 35 a on the light path of the laser light L₆ as shownin FIG. 20A to allow the light beam L₆ to pass by without splitting it.

The diffraction grating plate 35 is supported on a piezoelectric elementor the like, for example, and the piezoelectric element is controlled bya drive circuit included in the servo circuit 109 to control theposition of the diffraction grating plate 35. Alternatively, thediffraction grating plate 35 may be supported by a mechanism including ascrew and motor, and the diffraction grating plate 35 be positionedunder the control of a drive circuit included in the servo circuit 109.

The ratio of light amount diffracted by the diffraction grating 35 b isset as follows:First-order light : Zero-order light: Negative first-order light=25%50%: 25%

Note that positive/negative second-order light and higher-orderdiffracted light are not taken in consideration herein for theconvenience of illustration and explanation.

In this case, it is possible to vary the light beam used to write orread information signals to or from the optical disk within a range of100 to 50%. In this case, the positive/negative first-order light can beused for other purposes of cross-talking canceling etc.

FIGS. 21A and 21B are perspective views of a yet another illustrativeexample of the second type of optical-coupling efficiency varyingelement. This optical-coupling efficiency varying element uses a liquidcrystal element 36 capable of changing the phase difference in the formof a diffraction grating to branch a light beam L₇.

The liquid crystal element 36 is configured as follows. Namely, each ofthe transparent electrodes shown in FIGS. 19A and 19B is divided into aplurality of electrodes and the resultant divisional electrodes areapplied with different voltages, respectively, or a part of the glasssubstrate also shown in FIGS. 19A and 19B is formed slanted to provide avariation in thickness of the liquid crystal layer, to thereby formgrating-like regions different in phase difference from each other.Thus, a diffraction grating variable in phase depth is implemented.

In the liquid crystal element 36, the ratio of diffracted light amountvaries from one phase depth (difference in phase difference) to another.So, it can be used as follows:

For writing information signals to the optical disk, the ratio ofdiffracted light amount is set as follows:First-order light : Zero-order light : Negative first-orderlight=5%:90%:5%

For reading information signals from the optical disk, the ratio ofdiffracted light amount is set as follows:First-order light: Zero-order light : Negative first-orderlight=25%:50%:25%

In the foregoing, the present invention has been described in detailconcerning certain preferred embodiments thereof as examples withreference to the accompanying drawings. However, it should be understoodby those ordinarily skilled in the art that the present invention is notlimited to the embodiments but can be modified in various manners,constructed alternatively or embodied in various other forms withoutdeparting from the scope and spirit thereof as set forth and defined inthe appended claims.

Industrial Applicability

As having been described in the foregoing, even with a small power ratioof the light source between the write and read modes, the laser noise inthe read mode can be suppressed to a sufficiently low level. Using alight source whose maximum rating of optical output power is relativelysmall, it is possible to assure accurate write and read of informationsignals.

According to the present invention, the optical-coupling efficiencyvarying means can be controlled correspondingly to the type of anoptical recording medium, discriminated by the medium typediscriminating means, to the recording surface of the optical recordingmedium, discriminated by the recording surface discriminating means orto the recording area of the optical recording medium, discriminated bythe recording area discriminating means, to optimize the write and/orread light pulse on the recording surface of the optical recordingmedium.

1. An optical head comprising: a light source; a light focusing meansfor focusing a light beams emitted from the light source onto an opticalrecording medium; a beam splitting means for making the light beamemitted from the light source and return light coming from the opticalrecording medium via the light focusing means travel along differentlight paths; a light detecting means for detecting the return lightcoming from the optical recording medium via the beams splitting means;and an optical-coupling efficiency varying means and optical-couplingefficiency detecting means, provided between the light source and beamsplitting means, the optical-coupling efficiency varying means being tovary an optical-coupling efficiency that is a ratio of an amount oflight focused on the optical recording medium with a total amount oflight emitted from the light source; and the optical-coupling efficiencydetecting means being to detect information corresponding to anoptical-coupling efficiency varied by the optical-coupling efficiencyvarying means.
 2. The optical head according to claim 1, wherein theoptical-coupling efficiency detecting means is a light detecting meansfor detecting a light beam having been made by the optical-couplingefficiency varying means to travel along a light path branched from alight path extending toward the optical recording medium.
 3. The opticalhead according to claim 1, wherein the optical-coupling efficiencyvarying means includes a liquid crystal element and a beam splittingmembrane.
 4. The optical head according to claim 1, further comprisingan optical power controlling means for detecting the power of the lightbeam for irradiation to the optical recording medium and controlling thepower of the light beam from the light source on the basis of thedetected optical power to keep constant the power of the light beam forirradiation to the optical recording medium, the optical-couplingefficiency detecting means being a light detecting means for detecting aportion of a light beam emitted from the light source, that is notincluded in a range of divergent angle in which the rest of the lightbeam is focused on the optical recording medium.
 5. The optical headaccording to claim 1, wherein: the light source is a semiconductor laserand includes an optical power controlling means for detecting the powerof the light beam for irradiation to the optical recording medium andcontrolling the power of the light beam from the light source on thebasis of the detected optical power to keep constant the power of thelight beam for irradiation to the optical recording medium; and theoptical-coupling efficiency detecting means being a light detectingmeans for detecting a light beam emitted from one side of a laser chipof the semiconductor laser that emits, from the other side of the laserchip, a light beam that is focused on the optical recording medium. 6.The optical head according to claim 1, wherein the optical-couplingefficiency varying means is a light-path branching means for splittingan incident light beam into at least two light beams and making thelight beams travel along two or more different light paths and variesthe optical-coupling efficiency by varying the ratio in amount of lightfrom one to another of the two or more light paths.
 7. The optical headaccording to claim 6, wherein the optical-coupling efficiency varyingmeans includes a liquid crystal element and beam-splitting membrane. 8.The optical head according to claim 6, wherein the optical-couplingefficiency varying means includes a liquid crystal element having anarea shaped in the form of a diffraction grating and whose phasedifference can be varied.
 9. The optical head according to claim 6,wherein the optical-coupling efficiency varying means includes adiffraction grating, and a diffraction grating moving means.
 10. Theoptical head according to claim 1, wherein the optical-couplingefficiency varying means includes a wave plate, wave plate moving means,and a beam-splitting membrane.
 11. The optical head according to claim1, wherein the optical-coupling efficiency varying means can vary thetransmittance or reflectance of a light beam to vary theoptical-coupling efficiency of the light beam.
 12. The optical headaccording to claim 11, wherein the optical-coupling efficiency varyingmeans includes a filter means for lowering the transmittance of thelight beam and a filter-means moving means.
 13. The optical headaccording to claim 11, wherein the optical-coupling efficiency varyingmeans includes a liquid crystal element capable of varying thetransmittance of the light beam.
 14. An optical recording mediumrecording and/or playback apparatus which writes or reads informationsignals to a selected one of at least two or more types of opticalrecording media different in optimum recording-optical power and/orreading optical power from each other, the apparatus including,according to the present invention, an optical head including: a lightsource; and a light focusing means for focusing a light beams emittedfrom the light source onto an optical recording medium. the optical headincluding: a beam splitting means for making the light beam emitted fromthe light source and return light coming from the optical recordingmedium via the light focusing means travel along different light paths;a light detecting means for detecting the return light coming from theoptical recording medium via the beams splitting means; and anoptical-coupling efficiency varying means, and an optical-couplingefficiency detecting means, provided between the light source and beamsplitting means, the optical-coupling efficiency varying means being tovary an optical-coupling efficiency that is a ratio of an amount oflight focused on the optical recording medium with a total amount oflight emitted from the light source; and the optical-coupling efficiencydetecting means being to detect information corresponding to anoptical-coupling efficiency varied by the optical-coupling efficiencyvarying means.
 15. The apparatus according to 14, wherein informationsignals are written to and/or read from a selected one of at least twoor more types of optical recording media different in optimum writingoptical power and/or reading optical power from each other.
 16. Theapparatus according to claim 15, further comprising an optical-couplingefficiency controlling means for controlling, on the basis of detectionfrom the optical-coupling efficiency detecting means, the variation ofthe optical-coupling efficiency by the optical-coupling efficiencydetecting means and the power of the light beam from the light source,the optical-coupling efficiency controlling means controlling theoptical-coupling efficiency varying means correspondingly to the type ofthe optical recording medium used.
 17. The apparatus according to claim14, wherein the state of the optical-coupling efficiency varying meansis checked on the basis of the result of detection from theoptical-coupling efficiency detecting means.
 18. The apparatus accordingto claim 17, wherein there is preset a reference value for switching thestate of the optical-coupling efficiency varying means from one toanother.
 19. The apparatus according to claim 18, wherein the referencevalue is variably set correspondingly to the optimum reading opticalpower.
 20. The apparatus according to claim 18, wherein it is determinedbased on a relation in magnitude between the result of detection fromthe optical-coupling efficiency detecting means and the reference valuewhen the optical-coupling efficiency varying means should be switchedfrom one state to another.
 21. The apparatus according to claim 18,wherein it is determined based on a relation in magnitude between theresult of detection from the optical-coupling efficiency detecting meansand the reference value and a variation per hour of the result ofdetection from the optical-coupling efficiency detecting means when theoptical-coupling efficiency varying means should be switched from onestate to another.
 22. The apparatus according to claim 17, wherein it isdetermined based on a variation per hour of the result of detection fromthe optical-coupling efficiency detecting means when theoptical-coupling efficiency varying means should be switched from onestate to another.
 23. The apparatus according to claim 14, wherein it isdetermined based on the result of detection from the optical-couplingefficiency detecting means when an operation of switching between writeand read modes should be started.
 24. The apparatus according to claim23, wherein there is preset a reference value for switching the state ofthe optical-coupling efficiency varying means from one to another. 25.The apparatus according to claim 24, wherein the reference value isvariably set correspondingly to the optimum reading optical power. 26.The apparatus according to claim 24, wherein it is determined based on arelation in magnitude between the result of detection from theoptical-coupling efficiency detecting means and the reference value whenthe optical-coupling efficiency varying means should be switched fromone state to another.
 27. The apparatus according to claim 24, whereinit is determined based on a relation in magnitude between the result ofdetection from the optical-coupling efficiency detecting means and thereference value and a variation per hour of the result of detection fromthe optical-coupling efficiency detecting means when theoptical-coupling efficiency varying means should be switched from onestate to another.
 28. The apparatus according to claim 23, wherein it isdetermined based on a variation per hour of the result of detection fromthe optical-coupling efficiency detecting means when theoptical-coupling efficiency varying means should be switched from onestate to another.
 29. The apparatus according to claim 14, wherein it ischecked based on the result of detection from the optical-couplingefficiency detecting means whether the operation of the optical-couplingefficiency varying means is normal or not.
 30. The apparatus accordingto claim 14, wherein in case the optical-coupling efficiency varyingmeans operates more rapidly to lower the optical-coupling efficiencythan to raise the latter, it is set to a standby state with theoptical-coupling efficiency being raised, while in case the meansoperates more rapidly to raise the optical-coupling efficiency than tolower the latter, it is set to the standby state with theoptical-coupling efficiency being lowered.
 31. The apparatus accordingto claim 14, wherein the optical-coupling efficiency detecting means isa light detecting means for detecting a light beam having been made bythe optical-coupling efficiency varying means to travel along a lightpath branched from a light path extending toward the optical recordingmedium.
 32. The optical head according to claim 14, wherein theoptical-coupling efficiency varying means includes a liquid crystalelement and a beam splitting membrane.
 33. The optical head according toclaim 14, further comprising an optical power controlling means fordetecting the power of the light beam for irradiation to the opticalrecording medium and controlling the power of the light beam from thelight source on the basis of the detected optical power to keep constantthe power of the light beam for irradiation to the optical recordingmedium, the optical-coupling efficiency detecting means being a lightdetecting means for detecting a portion of a light beam emitted from thelight source, that is not included in a range of divergent angle inwhich the rest of the light beam is focused on the optical recordingmedium.
 34. The optical head according to claim 14, wherein: the lightsource is a semiconductor laser and includes an optical powercontrolling means for detecting the power of the light beam forirradiation to the optical recording medium and controlling the power ofthe light beam from the light source on the basis of the detectedoptical power to keep constant the power of the light beam forirradiation to the optical recording medium; and the optical-couplingefficiency detecting means being a light detecting means for detecting alight beam emitted from one side of a laser chip of the semiconductorlaser that emits, from the other side of the laser chip, a light beamthat is focused on the optical recording medium.
 35. The apparatusaccording to claim 14, wherein when writing information signals to anoptical recording medium of one type for which a writing optical powersmaller than an optimum writing optical power for an optical recordingmedium of any other type is optimum, the optical-coupling efficiencycontrolling means controls the optical-coupling efficiency to be smallerthan that with which information signals are written to the opticalrecording medium of the other type, and when reading information signalsfrom the optical recording medium of the one type for which a readingoptical power smaller than an optimum reading optical power for theoptical recording medium of the other type is optimum, theoptical-coupling efficiency controlling means the optical-couplingefficiency to be smaller than that with which information signals areread from the optical recording medium of the other type.
 36. Theapparatus according to claim 15, wherein the two or more types ofoptical recording media are different in optimum writing power and/orreading power of light on a recording surface from each other because ofa difference in relative speed between the optical head and opticalrecording media.
 37. The apparatus according to claim 15, wherein thetwo or more types of optical recording media are different in optimumwriting optical power and/or reading optical power on a recordingsurface from each other because of a difference in recording method fromone to another of the media.
 38. The apparatus according to claim 15,wherein each of the two or more types of optical recording medium is arecording surface of a multilayer optical recording medium having atleast two or more recording surfaces.
 39. The apparatus according toclaim 15, wherein at least one of the two or more types of opticalrecording media is a recording surface of a multilayer optical recordingmedium having at least two or more recording surfaces.
 40. The apparatusaccording to claim 15, wherein each of the two or more types of opticalrecording medium is a recording area of an optical recording mediumwhose recording surface is divided in at least two or more recordingareas.
 41. The apparatus according to claim 15, wherein at least one ofthe two or more types of optical recording medium is a recording area ofan optical recording medium whose recording surface is divided in atleast two or more recording areas.
 42. The apparatus according to claim15, further comprising a medium type discriminating means fordiscriminating the type of an optical recording medium, theoptical-coupling efficiency controlling means controlling theoptical-coupling efficiency varying means correspondingly to the type ofan optical recording medium, discriminated by the medium typediscriminating means.
 43. The apparatus according to claim 42, whereinthe medium type discriminating means discriminates the type of anoptical recording medium on the basis of the result of readingtable-of-contents information from the optical recording medium.
 44. Theapparatus according to claim 42, wherein the medium type discriminatingmeans discriminates the type of the optical recording medium on thebasis of the shape of an optical recording medium.
 45. The apparatusaccording to claim 42, wherein the medium type discriminating meansdiscriminates the type of an optical recording medium depending uponwhich one of recording layers in a multilayer optical recording mediumis the optical recording medium.
 46. The apparatus according to claim42, wherein the medium type discriminating means discriminates the typeof an optical recording medium depending upon which one of a pluralityof recording areas is the optical recording medium.
 47. The apparatusaccording to claim 42, wherein an optical-coupling efficiency isdetermined on the basis of a combination of a writing optical power andreading optical power for an optical recording medium having the typethereof discriminated by the medium type discrimination means as well asof a range of output in which the laser source output can be used. 48.The apparatus according to claim 47, wherein it is determined on thebasis of the determined optical-coupling efficiency whether theoptical-coupling efficiency should be switched from one to another atthe time of switching between write and read modes.
 49. The apparatusaccording to claim 42, wherein the optical-coupling efficiencycontrolling means controls the optical-coupling efficiency whilemonitoring the result of detection from the optical-coupling efficiencydetecting means on the basis of a combination of the result ofdiscrimination from the medium type discriminating means and a selectedmode of operation.
 50. The apparatus according to claim 14, wherein theoptical-coupling efficiency controlling means controls theoptical-coupling efficiency for optical recording media of the same typeto be smaller in the read mode than in the write mode.
 51. The apparatusaccording to claim 50, further comprising an optical-coupling efficiencycontrolling means for controlling, on the basis of detection from theoptical-coupling efficiency detecting means, the optical-couplingefficiency variation by the optical-coupling efficiency detecting meansand the power of the light beam from the light source, theoptical-coupling efficiency controlling means controlling, for switchingthe mode of operation from read to write, the optical-couplingefficiency varying means to vary the optical-coupling efficiency beforethe amount of light focused on the optical recording medium varies, andfor switching the mode of operation from write to read, theoptical-coupling efficiency varying means to vary the optical-couplingefficiency after the amount of light focused on the optical recordingmedium varies.
 52. The apparatus according to claim 14, wherein theoptical-coupling efficiency varying means is a light-path branchingmeans for splitting an incident light beam into at least two light beamsand making the light beams travel along two or more different lightpaths and varies the optical-coupling efficiency by varying the ratio inamount of light from one to another of the two or more light paths. 53.The apparatus according to claim 52, wherein the optical-couplingefficiency varying means includes a liquid crystal element andbeam-splitting membrane.
 54. The apparatus according to claim 52,wherein the optical-coupling efficiency varying means includes a liquidcrystal element having an area shaped in the form of a diffractiongrating and whose phase difference can be varied.
 55. The apparatusaccording to claim 52, wherein the optical-coupling efficiency varyingmeans includes a diffraction grating, and a diffraction grating movingmeans.
 56. The apparatus according to claim 14, wherein theoptical-coupling efficiency varying means includes a wave plate, waveplate moving means, and a beam-splitting membrane.
 57. The apparatusaccording to claim 14, wherein the optical-coupling efficiency varyingmeans can vary the transmittance or reflectance of a light beam to varythe optical-coupling efficiency of the light beam.
 58. The apparatusaccording to claim 57, wherein the optical-coupling efficiency varyingmeans includes a filter means for lowering the transmittance of thelight beam and a filter-means moving means.
 59. The apparatus accordingto claim 57, wherein the optical-coupling efficiency varying meansincludes a liquid crystal element capable of varying the transmittanceof the light beam.
 60. An optical recording medium recording and/orplayback method of writing and/or reading information signals to aselected one of at least two or more types of optical recording mediadifferent in optimum writing optical power and/or reading optical powerfrom each other, in which there is detected an optical-couplingefficiency, that is a ratio of an amount of light focused on the opticalrecording medium with a total amount of light emitted from the lightsource, and the optical-coupling efficiency is varied on the basis ofthe result of detection.
 61. The method according to claim 60, whereininformation signals are written to and/or read from a selected one of atleast two or more types of optical recording media different in optimumwriting optical power and/or reading optical power from each other. 62.The method according to claim 60, wherein when writing informationsignals to an optical recording medium of one type for which a writingoptical power smaller than an optimum writing optical power for anoptical recording medium of any other type is optimum, theoptical-coupling efficiency controlling means controls theoptical-coupling efficiency to be smaller than that with whichinformation signals are written to the optical recording medium of theother type, and when reading information signals from the opticalrecording medium of the one type for which a reading optical powersmaller than an optimum reading optical power for the optical recordingmedium of the other type is optimum, the optical-coupling efficiencycontrolling means the optical-coupling efficiency to be smaller thanthat with which information signals are read from the optical recordingmedium of the other type.
 63. The method according to claim 60, whereinthe optical-coupling efficiency controlling means controls theoptical-coupling efficiency for optical recording media of the same typeto be smaller in the write mode than in the read mode.
 64. The methodaccording to claim 63, further comprising an optical-coupling efficiencycontrolling means for controlling, on the basis of detection from theoptical-coupling efficiency detecting means, the optical-couplingefficiency variation by the optical-coupling efficiency detecting meansand the power of the light beam from the light source, theoptical-coupling efficiency controlling means controlling, for switchingthe mode of operation from read to write, the optical-couplingefficiency varying means to vary the optical-coupling efficiency beforethe amount of light focused on the optical recording medium varies, andfor switching the mode of operation from write to read, theoptical-coupling efficiency varying means to vary the optical-couplingefficiency after the amount of light focused on the optical recordingmedium varies.
 65. The method according to claim 60, wherein the stateof the optical-coupling efficiency varying means is checked on the basisof the result of detection from the optical-coupling efficiencydetecting means.
 66. The method according to claim 65, wherein there ispreset a reference value for switching the state of the optical-couplingefficiency varying means from one to another.
 67. The method accordingto claim 66, wherein the reference value is variably set correspondinglyto the optimum reading optical power.
 68. The method according to claim66, wherein it is determined based on a relation in magnitude betweenthe result of detection from the optical-coupling efficiency detectingmeans and the reference value when the optical-coupling efficiencyvarying means should be switched from one state to another.
 69. Themethod according to claim 66, wherein it is determined based on arelation in magnitude between the result of detection from theoptical-coupling efficiency detecting means and the reference value anda variation per hour of the result of detection from theoptical-coupling efficiency detecting means when the optical-couplingefficiency varying means should be switched from one state to another.70. The method according to claim 65, wherein it is determined based ona variation per hour of the result of detection from theoptical-coupling efficiency detecting means when the optical-couplingefficiency varying means should be switched from one state to another.71. The method according to claim 60, wherein it is determined based onthe result of detection from the optical-coupling efficiency detectingmeans when an operation of switching between write and read modes shouldbe started.
 72. The method according to claim 71, wherein there ispreset a reference value for switching the state of the optical-couplingefficiency varying means from one to another.
 73. The method accordingto claim 72, wherein the reference value is variably set correspondinglyto the optimum reading optical power.
 74. The method according to claim72, wherein it is determined based on a relation in magnitude betweenthe result of detection from the optical-coupling efficiency detectingmeans and the reference value when the optical-coupling efficiencyvarying means should be switched from one state to another.
 75. Themethod according to claim 72, wherein it is determined based on arelation in magnitude between the result of detection from theoptical-coupling efficiency detecting means and the reference value anda variation per hour of the result of detection from theoptical-coupling efficiency detecting means when the optical-couplingefficiency varying means should be switched from one state to another.76. The method according to claim 71, wherein it is determined based ona variation per hour of the result of detection from theoptical-coupling efficiency detecting means when the optical-couplingefficiency varying means should be switched from one state to another.77. The method according to claim 60, wherein it is checked based on theresult of detection from the optical-coupling efficiency detecting meanswhether the operation of the optical-coupling efficiency varying meansis normal or not.
 78. The method according to claim 60, wherein in casethe optical-coupling efficiency varying means operates more rapidly tolower the optical-coupling efficiency than to raise the latter, it isset to a standby state with the optical-coupling efficiency beingraised, while in case the means operates more rapidly to raise theoptical-coupling efficiency than to lower the latter, it is set to thestandby state with the optical-coupling efficiency being lowered. 79.The method according to claim 61, wherein the two or more types ofoptical recording media are different in optimum writing power and/orreading power of light on a recording surface from each other because ofa difference in relative speed between the optical head and opticalrecording media.
 80. The method according to claim 61, wherein the twoor more types of optical recording media are different in optimumwriting optical power and/or reading optical power on a recordingsurface from each other because of a difference in recording method fromone to another of the media.
 81. The method according to claim 61,wherein each of the two or more types of optical recording medium is arecording surface of a multilayer optical recording medium having atleast two or more recording surfaces.
 82. The method according to claim61, wherein at least one of the two or more types of optical recordingmedia is a recording surface of a multilayer optical recording mediumhaving at least two or more recording surfaces.
 83. The method accordingto claim 61, wherein each of the two or more types of optical recordingmedium is a recording area of an optical recording medium whoserecording surface is divided in at least two or more recording areas.84. The method according to claim 61, wherein at least one of the two ormore types of optical recording medium is a recording area of an opticalrecording medium whose recording surface is divided in at least two ormore recording areas.
 85. The method according to claim 61, wherein anoptical-coupling efficiency is determined on the basis of a combinationof a writing optical power and reading optical power for an opticalrecording medium used as well as of a range of output in which the lasersource output can be used.
 86. The method according to claim 85, whereinit is determined on the basis of the determined optical-couplingefficiency whether the optical-coupling efficiency should be switchedfrom one state to another at the time of switching between write andread modes.
 87. The method according to claim 60, wherein theoptical-coupling efficiency is controlled on the basis of a combinationof the type of the optical recording medium and a mode of operationselected.